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.