Resolved issue rendering multiple donut charts

After the innerRadius value is set, somehow it carries over to all other '.flot-overlay' canvases in the DOM as they are redrawn. So, inside drawOverlay() at line 715, I check this particular instance's options to determine whether we really do need to drawDonutHole():
if (options.series.pie.innerRadius > 0) {
    drawDonutHole(octx);	
}