PackedCircle

### Feature Proposal

I have a need for a PackedCircle chart - similar to a TreeMap but arbitrary sized circles clustered around a center (my use case only requires a single level rather than nested)

have found them supported in a number of other libraries, and while they are not as space efficient as treemaps, it's something I keep getting requests to support in the internal dashboard I maintain

### Possible Implementation

had started a discussion on https://github.com/kurkle/chartjs-chart-treemap/issues/15 and as well as the packing solution they use found this https://stackoverflow.com/a/21612877/1569675 (not DS3 specific, just a canvas implementation). As an interim, I might experiment with using this logic and then using it to populate a bubble chart, though it doesn't assign a 'center of gravity' to the middle of the box which would be a more desirable behavior