add to graphics chapter

[DaveBackus]

Known problems with Jupyter and downloading notebooks, Add to start of chapter.

• Notebook has .txt extension. Solution: In Jupyter, navigate to the file and rename it, removing the extension.
• Windows 10 + Chrome sometimes has no right-click Save as option. Solution: Use a different browser.
• Some foreign Macs won't run Jupyter. Solution: Ask for help. More at StackOverflow

[DaveBackus]

Put the last one in the intro chapter -- catch problems up front.

[DaveBackus]

An issue for near the end:

stress you can apply plot methods to series as well as df's
Add examples that graph variables one at a time with the same ax
Practice exam has one in which we do that to make one line thicker (it seems lw won't take a list)
Compare to creating a df with a subset of variables (only the ones we want to graph)

[DaveBackus]

Another thing to add: using explicit x and y parameters in the plot method.