1、问题详情，运行之后结果提示：

ModuleNotFoundError: No module named 'matplotlib'

2、解决方案：

原因就是缺少matplotlib lib包，导致不能识别函数，所以解决办法即时按照函数库

比较有用的是：

主要是有俩条命令。cmd执行：

python -m pip install matplotlib 

python -m pip install --upgrade pip

如下运行命令：


D:\root\python-Eclipse>python -m pip install --upgrade pip
Requirement already up-to-date: pip in d:\root\python\lib\site-packages (19.3.1)

D:\root\python-Eclipse>python -m pip install -U pip
Requirement already up-to-date: pip in d:\root\python\lib\site-packages (19.3.1)

D:\root\python-Eclipse>python -m pip install -U matplotlib

结果效果图：

Microsoft Windows [版本 10.0.17134.950]
(c) 2018 Microsoft Corporation。保留所有权利。

D:\root\python-Eclipse>pip install -U matplotlib
Retrying (Retry(total=4, connect=None, read=None, redirect=None, status=None)) after connection broken by 'ReadTimeoutError("HTTPSConnectionPool(host='pypi.org', port=443): Read timed out. (read timeout=15)")': /simple/matplotlib/
Requirement already up-to-date: matplotlib in c:\users\wb-ml590861\appdata\local\programs\python\python37-32\lib\site-packages (3.1.2)
Requirement already satisfied, skipping upgrade: cycler>=0.10 in c:\users\wb-ml590861\appdata\local\programs\python\python37-32\lib\site-packages (from matplotlib) (0.10.0)
Requirement already satisfied, skipping upgrade: kiwisolver>=1.0.1 in c:\users\wb-ml590861\appdata\local\programs\python\python37-32\lib\site-packages (from matplotlib) (1.1.0)
Requirement already satisfied, skipping upgrade: python-dateutil>=2.1 in c:\users\wb-ml590861\appdata\local\programs\python\python37-32\lib\site-packages (from matplotlib) (2.8.1)
Requirement already satisfied, skipping upgrade: numpy>=1.11 in c:\users\wb-ml590861\appdata\local\programs\python\python37-32\lib\site-packages (from matplotlib) (1.17.4)
Requirement already satisfied, skipping upgrade: pyparsing!=2.0.4,!=2.1.2,!=2.1.6,>=2.0.1 in c:\users\wb-ml590861\appdata\local\programs\python\python37-32\lib\site-packages (from matplotlib) (2.4.5)
Requirement already satisfied, skipping upgrade: six in c:\users\wb-ml590861\appdata\local\programs\python\python37-32\lib\site-packages (from cycler>=0.10->matplotlib) (1.13.0)
Requirement already satisfied, skipping upgrade: setuptools in c:\users\wb-ml590861\appdata\local\programs\python\python37-32\lib\site-packages (from kiwisolver>=1.0.1->matplotlib) (40.8.0)

D:\root\python-Eclipse>pip install matplotlib
Requirement already satisfied: matplotlib in c:\users\wb-ml590861\appdata\local\programs\python\python37-32\lib\site-packages (3.1.2)
Requirement already satisfied: python-dateutil>=2.1 in c:\users\wb-ml590861\appdata\local\programs\python\python37-32\lib\site-packages (from matplotlib) (2.8.1)
Requirement already satisfied: pyparsing!=2.0.4,!=2.1.2,!=2.1.6,>=2.0.1 in c:\users\wb-ml590861\appdata\local\programs\python\python37-32\lib\site-packages (from matplotlib) (2.4.5)
Requirement already satisfied: cycler>=0.10 in c:\users\wb-ml590861\appdata\local\programs\python\python37-32\lib\site-packages (from matplotlib) (0.10.0)
Requirement already satisfied: numpy>=1.11 in c:\users\wb-ml590861\appdata\local\programs\python\python37-32\lib\site-packages (from matplotlib) (1.17.4)
Requirement already satisfied: kiwisolver>=1.0.1 in c:\users\wb-ml590861\appdata\local\programs\python\python37-32\lib\site-packages (from matplotlib) (1.1.0)
Requirement already satisfied: six>=1.5 in c:\users\wb-ml590861\appdata\local\programs\python\python37-32\lib\site-packages (from python-dateutil>=2.1->matplotlib) (1.13.0)
Requirement already satisfied: setuptools in c:\users\wb-ml590861\appdata\local\programs\python\python37-32\lib\site-packages (from kiwisolver>=1.0.1->matplotlib) (40.8.0)
You are using pip version 19.0.3, however version 19.3.1 is available.
You should consider upgrading via the 'python -m pip install --upgrade pip' command.

D:\root\python-Eclipse>
D:\root\python-Eclipse>python -m pip install --upgrade pip
Requirement already up-to-date: pip in d:\root\python\lib\site-packages (19.3.1)

D:\root\python-Eclipse>python
Python 3.8.0 (tags/v3.8.0:fa919fd, Oct 14 2019, 19:37:50) [MSC v.1916 64 bit (AMD64)] on win32
Type "help", "copyright", "credits" or "license" for more information.
>>> import matplotlib.pyplot as plt
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ModuleNotFoundError: No module named 'matplotlib'
>>> import matplotlib.pyplot as plt
>>> input_values = [1, 2, 3, 4, 5]
>>> squares = [1, 4, 9, 16, 25]
>>> plt.plot(input_values, squares, linewidth=5)
[<matplotlib.lines.Line2D object at 0x000002BAE33F59A0>]
>>>
... plt.title("Square Numbers", fontsize=24)
Text(0.5, 1.0, 'Square Numbers')
>>> plt.xlabel("Value", fontsize=14)
Text(0.5, 0, 'Value')
>>> plt.ylabel("Square of Value", fontsize=14)
Text(0, 0.5, 'Square of Value')
>>>
... plt.tick_params(axis='both', labelsize=12)
>>> plt.show()

第一次没经验，导致运行的命令是：

Microsoft Windows [版本 10.0.17134.950]
(c) 2018 Microsoft Corporation。保留所有权利。

D:\root\python-Eclipse>pip install matplotlib
Requirement already satisfied: matplotlib in c:\users\wb-ml590861\appdata\local\programs\python\python37-32\lib\site-packages (3.1.2)
Requirement already satisfied: kiwisolver>=1.0.1 in c:\users\wb-ml590861\appdata\local\programs\python\python37-32\lib\site-packages (from matplotlib) (1.1.0)
Requirement already satisfied: numpy>=1.11 in c:\users\wb-ml590861\appdata\local\programs\python\python37-32\lib\site-packages (from matplotlib) (1.17.4)
Requirement already satisfied: pyparsing!=2.0.4,!=2.1.2,!=2.1.6,>=2.0.1 in c:\users\wb-ml590861\appdata\local\programs\python\python37-32\lib\site-packages (from matplotlib) (2.4.5)
Requirement already satisfied: cycler>=0.10 in c:\users\wb-ml590861\appdata\local\programs\python\python37-32\lib\site-packages (from matplotlib) (0.10.0)
Requirement already satisfied: python-dateutil>=2.1 in c:\users\wb-ml590861\appdata\local\programs\python\python37-32\lib\site-packages (from matplotlib) (2.8.1)
Requirement already satisfied: setuptools in c:\users\wb-ml590861\appdata\local\programs\python\python37-32\lib\site-packages (from kiwisolver>=1.0.1->matplotlib) (40.8.0)
Requirement already satisfied: six in c:\users\wb-ml590861\appdata\local\programs\python\python37-32\lib\site-packages (from cycler>=0.10->matplotlib) (1.13.0)
You are using pip version 19.0.3, however version 19.3.1 is available.
You should consider upgrading via the 'python -m pip install --upgrade pip' command.

D:\root\python-Eclipse>python -m pip install --upgrade pip
Requirement already up-to-date: pip in d:\root\python\lib\site-packages (19.3.1)

D:\root\python-Eclipse>python -m pip install -U pip
Requirement already up-to-date: pip in d:\root\python\lib\site-packages (19.3.1)

D:\root\python-Eclipse>python -m pip install -U matplotlib
Collecting matplotlib
  Downloading https://files.pythonhosted.org/packages/d0/7e/632e6ab64b93148b79da35e53d572c50c4a28d3964362e41228e8dd3bf93/matplotlib-3.1.2-cp38-cp38-win_amd64.whl (9.1MB)
     |████████████████████████████████| 9.1MB 930kB/s
Collecting kiwisolver>=1.0.1
  Downloading https://files.pythonhosted.org/packages/73/c8/ec7ec2a9b5efd26cdb5a72bfff1cfc2eda11d981bb58a44351c7113e2e29/kiwisolver-1.1.0-cp38-none-win_amd64.whl (57kB)
     |████████████████████████████████| 61kB 2.0MB/s
Collecting python-dateutil>=2.1
  Downloading https://files.pythonhosted.org/packages/d4/70/d60450c3dd48ef87586924207ae8907090de0b306af2bce5d134d78615cb/python_dateutil-2.8.1-py2.py3-none-any.whl (227kB)
     |████████████████████████████████| 235kB ...
Collecting cycler>=0.10
  Downloading https://files.pythonhosted.org/packages/f7/d2/e07d3ebb2bd7af696440ce7e754c59dd546ffe1bbe732c8ab68b9c834e61/cycler-0.10.0-py2.py3-none-any.whl
Collecting pyparsing!=2.0.4,!=2.1.2,!=2.1.6,>=2.0.1
  Downloading https://files.pythonhosted.org/packages/c0/0c/fc2e007d9a992d997f04a80125b0f183da7fb554f1de701bbb70a8e7d479/pyparsing-2.4.5-py2.py3-none-any.whl (67kB)
     |████████████████████████████████| 71kB 2.3MB/s
Collecting numpy>=1.11
  Downloading https://files.pythonhosted.org/packages/ca/11/c81d07e47d197634ac175941bf0de5add37d40a6b9e9a79723fae7380e56/numpy-1.17.4-cp38-cp38-win_amd64.whl (12.7MB)
     |████████████████████████████████| 12.7MB 656kB/s
Requirement already satisfied, skipping upgrade: setuptools in d:\root\python\lib\site-packages (from kiwisolver>=1.0.1->matplotlib) (41.2.0)
Collecting six>=1.5
  Downloading https://files.pythonhosted.org/packages/65/26/32b8464df2a97e6dd1b656ed26b2c194606c16fe163c695a992b36c11cdf/six-1.13.0-py2.py3-none-any.whl
Installing collected packages: kiwisolver, six, python-dateutil, cycler, pyparsing, numpy, matplotlib
  WARNING: The script f2py.exe is installed in 'D:\root\python\Scripts' which is not on PATH.
  Consider adding this directory to PATH or, if you prefer to suppress this warning, use --no-warn-script-location.
Successfully installed cycler-0.10.0 kiwisolver-1.1.0 matplotlib-3.1.2 numpy-1.17.4 pyparsing-2.4.5 python-dateutil-2.8.1 six-1.13.0

D:\root\python-Eclipse>Microsoft Windows [版本 10.0.17134.950]
'Microsoft' 不是内部或外部命令，也不是可运行的程序
或批处理文件。

D:\root\python-Eclipse>(c) 2018 Microsoft Corporation。保留所有权利。
此时不应有 2018。

D:\root\python-Eclipse>
D:\root\python-Eclipse>D:\root\python-Eclipse>pip install -U matplotlib
'D:\root\python-Eclipse' 不是内部或外部命令，也不是可运行的程序
或批处理文件。

D:\root\python-Eclipse>Retrying (Retry(total=4, connect=None, read=None, redirect=None, status=None)) after connection broken by 'ReadTimeoutError("HTTPSConnectionPool(host='pypi.org', port=443): Read timed out. (read timeout=15)")': /simple/matplotlib/
'Retrying' 不是内部或外部命令，也不是可运行的程序
或批处理文件。

D:\root\python-Eclipse>Requirement already up-to-date: matplotlib in c:\users\wb-ml590861\appdata\local\programs\python\python37-32\lib\site-packages (3.1.2)
'Requirement' 不是内部或外部命令，也不是可运行的程序
或批处理文件。

D:\root\python-Eclipse>Requirement already satisfied, skipping upgrade: cycler>=0.10 in c:\users\wb-ml590861\appdata\local\programs\python\python37-32\lib\site-packages (from matplotlib) (0.10.0)
'Requirement' 不是内部或外部命令，也不是可运行的程序
或批处理文件。

D:\root\python-Eclipse>Requirement already satisfied, skipping upgrade: kiwisolver>=1.0.1 in c:\users\wb-ml590861\appdata\local\programs\python\python37-32\lib\site-packages (from matplotlib) (1.1.0)
'Requirement' 不是内部或外部命令，也不是可运行的程序
或批处理文件。

D:\root\python-Eclipse>Requirement already satisfied, skipping upgrade: python-dateutil>=2.1 in c:\users\wb-ml590861\appdata\local\programs\python\python37-32\lib\site-packages (from matplotlib) (2.8.1)
'Requirement' 不是内部或外部命令，也不是可运行的程序
或批处理文件。

D:\root\python-Eclipse>Requirement already satisfied, skipping upgrade: numpy>=1.11 in c:\users\wb-ml590861\appdata\local\programs\python\python37-32\lib\site-packages (from matplotlib) (1.17.4)
'Requirement' 不是内部或外部命令，也不是可运行的程序
或批处理文件。

D:\root\python-Eclipse>Requirement already satisfied, skipping upgrade: pyparsing!=2.0.4,!=2.1.2,!=2.1.6,>=2.0.1 in c:\users\wb-ml590861\appdata\local\programs\python\python37-32\lib\site-packages (from matplotlib) (2.4.5)
'Requirement' 不是内部或外部命令，也不是可运行的程序
或批处理文件。

D:\root\python-Eclipse>Requirement already satisfied, skipping upgrade: six in c:\users\wb-ml590861\appdata\local\programs\python\python37-32\lib\site-packages (from cycler>=0.10->matplotlib) (1.13.0)
'Requirement' 不是内部或外部命令，也不是可运行的程序
或批处理文件。

D:\root\python-Eclipse>Requirement already satisfied, skipping upgrade: setuptools in c:\users\wb-ml590861\appdata\local\programs\python\python37-32\lib\site-packages (from kiwisolver>=1.0.1->matplotlib) (40.8.0)
'Requirement' 不是内部或外部命令，也不是可运行的程序
或批处理文件。

D:\root\python-Eclipse>
D:\root\python-Eclipse>D:\root\python-Eclipse>pip install matplotlib
'D:\root\python-Eclipse' 不是内部或外部命令，也不是可运行的程序
或批处理文件。

D:\root\python-Eclipse>Requirement already satisfied: matplotlib in c:\users\wb-ml590861\appdata\local\programs\python\python37-32\lib\site-packages (3.1.2)
'Requirement' 不是内部或外部命令，也不是可运行的程序
或批处理文件。

D:\root\python-Eclipse>Requirement already satisfied: python-dateutil>=2.1 in c:\users\wb-ml590861\appdata\local\programs\python\python37-32\lib\site-packages (from matplotlib) (2.8.1)
'Requirement' 不是内部或外部命令，也不是可运行的程序
或批处理文件。

D:\root\python-Eclipse>Requirement already satisfied: pyparsing!=2.0.4,!=2.1.2,!=2.1.6,>=2.0.1 in c:\users\wb-ml590861\appdata\local\programs\python\python37-32\lib\site-packages (from matplotlib) (2.4.5)
'Requirement' 不是内部或外部命令，也不是可运行的程序
或批处理文件。

D:\root\python-Eclipse>Requirement already satisfied: cycler>=0.10 in c:\users\wb-ml590861\appdata\local\programs\python\python37-32\lib\site-packages (from matplotlib) (0.10.0)
'Requirement' 不是内部或外部命令，也不是可运行的程序
或批处理文件。

D:\root\python-Eclipse>Requirement already satisfied: numpy>=1.11 in c:\users\wb-ml590861\appdata\local\programs\python\python37-32\lib\site-packages (from matplotlib) (1.17.4)
'Requirement' 不是内部或外部命令，也不是可运行的程序
或批处理文件。

D:\root\python-Eclipse>Requirement already satisfied: kiwisolver>=1.0.1 in c:\users\wb-ml590861\appdata\local\programs\python\python37-32\lib\site-packages (from matplotlib) (1.1.0)
'Requirement' 不是内部或外部命令，也不是可运行的程序
或批处理文件。

D:\root\python-Eclipse>Requirement already satisfied: six>=1.5 in c:\users\wb-ml590861\appdata\local\programs\python\python37-32\lib\site-packages (from python-dateutil>=2.1->matplotlib) (1.13.0)
'Requirement' 不是内部或外部命令，也不是可运行的程序
或批处理文件。

D:\root\python-Eclipse>Requirement already satisfied: setuptools in c:\users\wb-ml590861\appdata\local\programs\python\python37-32\lib\site-packages (from kiwisolver>=1.0.1->matplotlib) (40.8.0)
'Requirement' 不是内部或外部命令，也不是可运行的程序
或批处理文件。

D:\root\python-Eclipse>You are using pip version 19.0.3, however version 19.3.1 is available.
'You' 不是内部或外部命令，也不是可运行的程序
或批处理文件。

D:\root\python-Eclipse>You should consider upgrading via the 'python -m pip install --upgrade pip' command.
'You' 不是内部或外部命令，也不是可运行的程序
或批处理文件。

D:\root\python-Eclipse>
D:\root\python-Eclipse>D:\root\python-Eclipse>
命令语法不正确。
D:\root\python-Eclipse>D:\root\python-Eclipse>python -m pip install --upgrade pip
'D:\root\python-Eclipse' 不是内部或外部命令，也不是可运行的程序
或批处理文件。

D:\root\python-Eclipse>Requirement already up-to-date: pip in d:\root\python\lib\site-packages (19.3.1)
'Requirement' 不是内部或外部命令，也不是可运行的程序
或批处理文件。

D:\root\python-Eclipse>
D:\root\python-Eclipse>D:\root\python-Eclipse>python
'D:\root\python-Eclipse' 不是内部或外部命令，也不是可运行的程序
或批处理文件。

D:\root\python-Eclipse>Python 3.8.0 (tags/v3.8.0:fa919fd, Oct 14 2019, 19:37:50) [MSC v.1916 64 bit (AMD64)] on win32
Python: can't open file '3.8.0': [Errno 2] No such file or directory

D:\root\python-Eclipse>Type "help", "copyright", "credits" or "license" for more information.
系统找不到指定的文件。
处理: help 时出错。
系统找不到指定的文件。
处理: copyright 时出错。
系统找不到指定的文件。
处理: credits 时出错。
系统找不到指定的文件。
处理: or 时出错。
系统找不到指定的文件。
处理: license 时出错。
系统找不到指定的文件。
处理: for 时出错。
系统找不到指定的文件。
处理: more 时出错。
系统找不到指定的文件。
处理: information. 时出错。

D:\root\python-Eclipse>>>> import matplotlib.pyplot as plt
此时不应有 >。

D:\root\python-Eclipse>Traceback (most recent call last):
'Traceback' 不是内部或外部命令，也不是可运行的程序
或批处理文件。

D:\root\python-Eclipse>  File "<stdin>", line 1, in <module>
命令语法不正确。

D:\root\python-Eclipse>ModuleNotFoundError: No module named 'matplotlib'
'ModuleNotFoundError:' 不是内部或外部命令，也不是可运行的程序
或批处理文件。

D:\root\python-Eclipse>>>> import matplotlib.pyplot as plt
此时不应有 >。

D:\root\python-Eclipse>>>> input_values = [1, 2, 3, 4, 5]
此时不应有 >。

D:\root\python-Eclipse>>>> squares = [1, 4, 9, 16, 25]
此时不应有 >。

D:\root\python-Eclipse>>>> plt.plot(input_values, squares, linewidth=5)
此时不应有 >。

D:\root\python-Eclipse>[<matplotlib.lines.Line2D object at 0x000002BAE33F59A0>]
系统找不到指定的文件。

D:\root\python-Eclipse>>>>
此时不应有 >。

D:\root\python-Eclipse>... plt.title("Square Numbers", fontsize=24)
'...' 不是内部或外部命令，也不是可运行的程序
或批处理文件。

D:\root\python-Eclipse>Text(0.5, 1.0, 'Square Numbers')
'Text' 不是内部或外部命令，也不是可运行的程序
或批处理文件。

D:\root\python-Eclipse>>>> plt.xlabel("Value", fontsize=14)
此时不应有 >。

D:\root\python-Eclipse>Text(0.5, 0, 'Value')
'Text' 不是内部或外部命令，也不是可运行的程序
或批处理文件。

D:\root\python-Eclipse>>>> plt.ylabel("Square of Value", fontsize=14)
此时不应有 >。

D:\root\python-Eclipse>Text(0, 0.5, 'Square of Value')
'Text' 不是内部或外部命令，也不是可运行的程序
或批处理文件。

D:\root\python-Eclipse>>>>
此时不应有 >。

D:\root\python-Eclipse>... plt.tick_params(axis='both', labelsize=12)
'...' 不是内部或外部命令，也不是可运行的程序
或批处理文件。

D:\root\python-Eclipse>>>> plt.show()
此时不应有 >。

D:\root\python-Eclipse>

运行出来的效果是：

 

参照这张图操作，可以解决这个问题:

命令在上面：

命令行复制出来如下：

Microsoft Windows [版本 10.0.17134.950]
(c) 2018 Microsoft Corporation。保留所有权利。

D:\root\python-Eclipse>pip install -U matplotlib
Retrying (Retry(total=4, connect=None, read=None, redirect=None, status=None)) after connection broken by 'ReadTimeoutError("HTTPSConnectionPool(host='pypi.org', port=443): Read timed out. (read timeout=15)")': /simple/matplotlib/
Requirement already up-to-date: matplotlib in c:\users\wb-ml590861\appdata\local\programs\python\python37-32\lib\site-packages (3.1.2)
Requirement already satisfied, skipping upgrade: cycler>=0.10 in c:\users\wb-ml590861\appdata\local\programs\python\python37-32\lib\site-packages (from matplotlib) (0.10.0)
Requirement already satisfied, skipping upgrade: kiwisolver>=1.0.1 in c:\users\wb-ml590861\appdata\local\programs\python\python37-32\lib\site-packages (from matplotlib) (1.1.0)
Requirement already satisfied, skipping upgrade: python-dateutil>=2.1 in c:\users\wb-ml590861\appdata\local\programs\python\python37-32\lib\site-packages (from matplotlib) (2.8.1)
Requirement already satisfied, skipping upgrade: numpy>=1.11 in c:\users\wb-ml590861\appdata\local\programs\python\python37-32\lib\site-packages (from matplotlib) (1.17.4)
Requirement already satisfied, skipping upgrade: pyparsing!=2.0.4,!=2.1.2,!=2.1.6,>=2.0.1 in c:\users\wb-ml590861\appdata\local\programs\python\python37-32\lib\site-packages (from matplotlib) (2.4.5)
Requirement already satisfied, skipping upgrade: six in c:\users\wb-ml590861\appdata\local\programs\python\python37-32\lib\site-packages (from cycler>=0.10->matplotlib) (1.13.0)
Requirement already satisfied, skipping upgrade: setuptools in c:\users\wb-ml590861\appdata\local\programs\python\python37-32\lib\site-packages (from kiwisolver>=1.0.1->matplotlib) (40.8.0)

D:\root\python-Eclipse>pip install matplotlib
Requirement already satisfied: matplotlib in c:\users\wb-ml590861\appdata\local\programs\python\python37-32\lib\site-packages (3.1.2)
Requirement already satisfied: python-dateutil>=2.1 in c:\users\wb-ml590861\appdata\local\programs\python\python37-32\lib\site-packages (from matplotlib) (2.8.1)
Requirement already satisfied: pyparsing!=2.0.4,!=2.1.2,!=2.1.6,>=2.0.1 in c:\users\wb-ml590861\appdata\local\programs\python\python37-32\lib\site-packages (from matplotlib) (2.4.5)
Requirement already satisfied: cycler>=0.10 in c:\users\wb-ml590861\appdata\local\programs\python\python37-32\lib\site-packages (from matplotlib) (0.10.0)
Requirement already satisfied: numpy>=1.11 in c:\users\wb-ml590861\appdata\local\programs\python\python37-32\lib\site-packages (from matplotlib) (1.17.4)
Requirement already satisfied: kiwisolver>=1.0.1 in c:\users\wb-ml590861\appdata\local\programs\python\python37-32\lib\site-packages (from matplotlib) (1.1.0)
Requirement already satisfied: six>=1.5 in c:\users\wb-ml590861\appdata\local\programs\python\python37-32\lib\site-packages (from python-dateutil>=2.1->matplotlib) (1.13.0)
Requirement already satisfied: setuptools in c:\users\wb-ml590861\appdata\local\programs\python\python37-32\lib\site-packages (from kiwisolver>=1.0.1->matplotlib) (40.8.0)
You are using pip version 19.0.3, however version 19.3.1 is available.
You should consider upgrading via the 'python -m pip install --upgrade pip' command.

D:\root\python-Eclipse>
D:\root\python-Eclipse>python -m pip install --upgrade pip
Requirement already up-to-date: pip in d:\root\python\lib\site-packages (19.3.1)

D:\root\python-Eclipse>python
Python 3.8.0 (tags/v3.8.0:fa919fd, Oct 14 2019, 19:37:50) [MSC v.1916 64 bit (AMD64)] on win32
Type "help", "copyright", "credits" or "license" for more information.
>>> import matplotlib.pyplot as plt
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ModuleNotFoundError: No module named 'matplotlib'
>>> import matplotlib.pyplot as plt
>>> input_values = [1, 2, 3, 4, 5]
>>> squares = [1, 4, 9, 16, 25]
>>> plt.plot(input_values, squares, linewidth=5)
[<matplotlib.lines.Line2D object at 0x000002BAE33F59A0>]
>>>
... plt.title("Square Numbers", fontsize=24)
Text(0.5, 1.0, 'Square Numbers')
>>> plt.xlabel("Value", fontsize=14)
Text(0.5, 0, 'Value')
>>> plt.ylabel("Square of Value", fontsize=14)
Text(0, 0.5, 'Square of Value')
>>>
... plt.tick_params(axis='both', labelsize=12)
>>> plt.show()

然后操作运行正常，出来了效果。

我发现使用Python来做数据可视化操作真的很香！

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