Support turning lines on and off by clicking corresponding legend items

[openjck]

This feature request came from Rob Miller:

one thing that would be awesome would be to have the ability to toggle lines on the graph by clicking them on the legend

For example, if someone clicks "Other", the population line for "Other" should disappear.

I don't think metrics-graphics supports this natively, but it may be possible to add this behavior without metrics-graphics doing it.

[openjck]

I requested this feature from metrics-graphics, but we can also add the feature on top of metrics-graphics ourselves.