Here is the XML file. The data is actually being created dynamically from within a flash file, but I traced out the XML strings used to create the chart and included them in the file.
One thing that I noticed is that, if I have values outside of my yAxisMinValue and yAxisMaxValue, the overlayed gradient does not appear, and shows the background gradient in full color. Its only when all values are within the -3, 3 range that I set that the overlay occurs.
sample.xml