I saw this question here: Coreplot graph fill color along y-axis
It answers how to fill the plot along the xaxis:
CPTPlotRange *range = [CPTPlotRange plotRangeWithLocation:CPTDecimalFromDouble(4.0) length:CPTDecimalFromDouble(6.0)];
CPTFill *bandFill = [CPTFill fillWithColor:[[CPTColor whiteColor] colorWithAlphaComponent:.2f]];
[xAxis addBackgroundLimitBand:[CPTLimitBand limitBandWithRange:range
fill:bandFill]];
However, that gives me a graph like this:

I'd like the white color to only be under the graph line, not above it.
Is there a way to do that?