No idea from looking at the code. What does your debugger say?

A couple of things though.

1) please learn to use code tags

2)
Code:
if ((displayPriceRangeLow < 1) || (displayPriceRangeLow == NULL))
{
displayPriceRangeHigh = 1;
displayPriceRangeLow = 1;
}
// get pixels per dollar
pixelsPerDollar = displayChartHeight / (displayPriceRangeHigh - displayPriceRangeLow);
You have potential division by zero there.

Also, typically NULL is used to test pointer values, not POD types. And since your first test is < 1, NULL would also be < 1, so even if testing for NULL was valid, it would be redundant.