Implement .elasticRadius for Bubble Overlay charts

[hypsypops]

Circles should resize as data is filtered in other charts using .elasticRadius([boolean]) as done in the standard bubble chart.

Seems like it would be simple, but I'm still a little noob-ish and haven't been able to successfully do it myself. :)

Thanks for a great library!

[gordonwoodhull]

Are you talking about

1. adjusting the scale of bubbles so that the smallest and largest are always the same size, no matter the data, or are you saying
2. the bubbles don't resize when filters are applied?

If you're saying 2., it seems to work on the crime example.

http://dc-js.github.io/dc.js/crime/

If you're saying 1., yes this would be very simple, as it's only a couple of lines of code that set the radius scale domain, here:
https://github.com/dc-js/dc.js/blob/master/src/bubble-chart.js#L63

But I'd prefer to implement the feature at the bubbleMixin level rather than duplicating it, and that will take some minor refactoring.

[hypsypops]

I'm saying number 1.

Currently, as you filter down through the data the bubbles get smaller and smaller, so you can't see any relationships between what's left. Relative sizing would fix that, and I think it's something other users may want in the future.

[gordonwoodhull]

Would gladly take a PR on this; I believe it is just a matter of moving some code up from bubble chart to bubble mixin, and adding the relevant tests to the bubble overlay.

[gordonwoodhull]

Implemented in dc.js 2.1.2