apply d3.nest rollup to nested pie chart (to vary the size of the pies)

apply d3.nest rollup to nested pie chart (to vary the size of the pies)

I'd like to modify Mike Bostock's nested pies example
http://bl.ocks.org/mbostock/1305337 to have different pie sizes per the
number of flights in each airport; to have a visual idea of how busy each
airport is. My guess is to sum up the number of flights via
.rollup(function(leaves) { return {"total_flights": d3.sum(leaves,
function(d) {return +(d.flights);})} }) (from
http://bl.ocks.org/phoebebright/raw/3176159/) and apply it somehow to r
(radius) but as I'm still learning, I don't know where to inject it.
Appreciate your tips/help.