Proportional circle areas?

[Dan]

How do I define a D3 radius scale so that the areas of the circles
will be proportional? (This should be easy but my scale is off by a
few pixels ... Perhaps it is a rounding issue ...?)

attr("r", function(d){ return rScale(d.value) })

[Mike Bostock]

> How do I define a D3 radius scale so that the areas of the circles
> will be proportional?

Generally by using a d3.scale.sqrt, and by setting the lower bound of
the domain and range to 0. For example:

var r = d3.scale.sqrt()
.domain([0, 5000])
.range([0, 20]);

Mike