Make everything a bit more pythonic and readable, as well as flipping the graph upside down so higher values are drawn above instead of below.
Pixel coordinates are 0 at the top of the screen, so to draw high graph values, a low pixel coordinate has to be used.
With only a little bit of effort, the drawing code can be made a lot more pythonic. `== True` and `== False` are never acceptable, and for loops simply don't execute if the iterator is empty - no need to check.
