encode data by color in bubbleChart

[gordonwoodhull]

Currently the color is used to encode the selection status.

But sometimes it's nice to encode some other property using color.

I'm not sure we want to go as way-out as dc.graph.js, where attributes are cascaded, and selection can set arbitrary properties. But maybe this could be more flexible.

[gordonwoodhull]

Also, the choropleth chart currently uses transparency for selection status, which is kind of nice.