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]
Data colors were implemented in lab-grapher even before, but Lab didn't support them, right?
Is color specified as an array because grapher expects that as well? I think it would be much nicer for authors to use HTML color definitions. However it doesn't have to be part of this pull request.