Ah yes! That got it to work!! While playing with it a bit more, I noticed that as long as I gave it an id, I could use the original code you linked to, where it was calling

var chartId = sprite.surface.id;
var chart = Ext.getCmp(chartId); // this works as long as I have some id defined in the chart object, even if I'm not calling it here as I was earlier.
You can play with the final results on jsfiddle here.

Thanks again!