contributors graph: Add support for putting more than one line on the plot

[jnareb]

Example could be 'n_mod' and Additions ('+:count'), or 'n_mod' and Deletions ('-:count').

To test:

• whether we can provide multiple colors for multiple columns in hvPlot

TODO

• theoretically we should compute max of max for yscale range...
• needs legend

[jnareb]

Partially solved by 827d5be (provides infrastructure)