Hover legend and interactive legend

Added examples of both a hoverLegend and interactiveLegend. The interactiveLegend could possibly replace both toggleLegend and hoverLegend, but at the moment, it feels a bit clunky, since all I did was copy code over to that class. 
this comment is inaccurate since it says it is the element being hovered over, but is actually typed as a string. rather than having a comment, could you just come up with a more descriptive variable name? like `datumCurrentlyFocusedOn`