Beginning to work on an example for dynamically updating the chart. However, some of these elements need to be refactored because the existing rects are not being cleared upon update.
Wrap all the various features accessors in the functor call to ensure that we can send in a simple value or a function. e.g.

chart.using('bars', function(bars){

   // non function values allowed
   bars.width(20)

   // or functions allowed
   bars.width(function(){return 20;});
});