Adding colormap demonstration to plotting lesson

[christopherlovell]

When teaching the plotting episode the only major omission that comes to mind is colormaps. This is one of the major means of showing a third data 'axis', and would provide another means for new users to abide by the accessibility recommendations at the end of the lesson.

The concept could be introduced after the first demonstration of scatter, by swapping the s size argument for c. This hopefully wouldn't make the lesson too long, as it's essentially a small change to an existing example.

[martinosorb]

@christopherlovell thanks - I know years have passed but I like the idea. Currently the s= parameter is used only in an exercise. Is that where you would like to use c= as well?