To diagnose, set a breakpoint where you're setting the axis width. When the breakpoint gets hit, make sure the size of the chart is something that makes sense. If it's very small, or zero, it may not actually be in the view hierarchy yet, and there's your problem.
For instance, you can't do this in init, it's better to do it somewhere like viewDidLoad.
Something like this:
SChartNumberRange *range = [[SChartNumberRange alloc] initWithMinimum:minY
yAxis = [[SChartNumberAxis alloc] initWithRange:range];
yAxis.width = [NSNumber numberWithInt:YAxisWidth];