#327 Multiple colors and strokes in LegendItem

[sergivs]

Sometimes it's convenient to have a legend item that describes several traces on the plot; this helps keep the legend less cluttered. If the traces don't have symbols but only lines, a legend item with several stacked lines of different colors would be useful. As far as I can see from the API, it's not possible to do that with the LegendItem class.

I would like to request the following changes:

Replace the LegendItem class with a more generic interface.

Add an implementation that supports multiple shapes and colors.

[sergivs]

Logged In: YES
user_id=390604
Originator: YES

It would also be useful to have an implementation that would support different line strokes.

Please note that this is not the same as having distinct line and outline strokes.