I am running windows 7 with Python version  3.7.1 and Pip version     19.1.1
I needed to install Microsoft visual studio.
Microsoft visual studio is needed for the kiwisolver that is installed with matplotlib using pip. It is free but a 4MB download took over one to two hours to get on my machine and installed.
After using pip to install matplotlib, but without visual studio on my machine, running my script with
import matplotlib.pyplot as plt
the error I had was: 
import matplotlib ImportError: DLL load failed: The specified procedure could not be found
After installing visual studio everything worked great!
Get Microsoft visual studio here.