Ability to remove (hide) legend or other visualization after it has been added

[drosen0]

Currently the way to do this is by manipulating the SVG (HTML for legend) or CSS.

Contour doesn’t have a defined way to interact with visualizations after they’ve been added. It’s limited to updating the dataset, setting options, and re-rendering. I could see implementing this a couple ways:

a. Add an option specific to the legend like enabled: <bool>. Code to add/remove the legend when enabled is toggled would be within legend's render().

b. Add VisInstanceContainer#setVisibility(<bool>) to toggle visibility on the container. Some visualizations, particularly the legend, add content outside the container. Therefore there would need to be an API for the visualization to listen for this change and add/remove itself.

cc @narenranjit