matplotlib questions

[Steve]

I am trying to modify the "Bar Graph Demo" at
https://matplotlib.org/stable/gallery/index.html,  Lines, bars, and markers
but the more I experiment and change the code, the more messed up it
becomes.  I have the demo code working. This is my second attempt.  I guess
I accidentally got my first chart working but this second one, not so good.

Is there any source to help me understand how the code works and how to
change it?
Among many others options, how do I change the font size? 
How are the labels placed on the X-axis and Y-axis?