Add legend to visual display

[hazelshapiro]

Indicate with a color key, similar to below, which colors correspond to which performance rating values.

• Should we use this colormap? No, Viridis

[mfisher87]

@hazelshapiro can you please clarify? A scale bar is something I imagine we'd see on a map, but not a sankey diagram, as the diagram has no real-world size.

[hazelshapiro]

This would be a key to say what colors correspond to what performance score

[mfisher87]

Got it. On the legacy site, we have the option to color by node type or value:

Will our first implementation be to color by value, and later add other coloring options?

What do you think about a different colormap for this? The current one is not accessible to those with color vision deficiency (CVD), and it would be easier to start out with an accessible colormap than to come back to it later.

[mfisher87]

We met and decided to start with replacing this colormap with Viridis to see how our expert committee likes it. Going forward, we may support selecting various colormaps.

[mfisher87]

@hazelshapiro with #282 do we still need this ticket? Or do we want the legend to be in this specific style? (in which case, let's re-title the ticket)