dispersion_plot error

Python version: 2.7
Windows version: Windows 7 64-bit
Language of the system: Chinese
In Natural Language Processing with Python, I meet an error in using function dispersion_plot
My code is in the following

ValueError('The plot function requires matplotlib to be installed.')

So I install this module matplotlib with tool pip(pip tool can be installed in https://pypi.python.org/pypi/pip/)
Using code below:

python -m pip install matplotlib

In this way, I solve this problem, now let’s see the result:

text4.dispersion_plot(["citizens","democracy","freedom","duties","America"])