Install python PIP
You can determine whether it is installed by using the following command:

pip --version     # Python2.x 
pip3 --version    # Python3.x 

If not already installed, Ubuntu can be installed using the following methods:

sudo apt-get install python-pip

Upgrade matplotlib
View the matplotlib version

pip show matplotlib

And then upgrade

pip install --upgrade matplotlib

Then run rqt_plot

rosrun rqt_plot rqt_plot