Graphs have customizable line colors and integrated legend

See https://github.com/concord-consortium/lab-grapher/pull/9
Graphs not only have customized line colors, but those colors are used as part the generated legend!
That's *two* merge requests for the price of *one*! :+1:

Also did some minor formatting and code cleanup.
[#74535922]
[#74536668]
Great, thanks! As I said, it's a side thing, but if it's easy to fix it, it would be nice.