Xcode considers an iPhone 4/4s to have an effective area size of 320 x 480 - if you're in landscape, a width of 500 would take you 20px outside of the visible screen area. The new iPod touch models have 320 x 568 retina screens and it's likely your simulator has defaulted to 4" iPhone of the same screen size.
OpenGL forbids the creation of a framebuffer for an area larger than the screen for the chart, as you've seen, limiting the size of the chart you can create. However, the chart itself is like a scrollview in that you can pan and zoom around. I like to think of a chart as a 'window on your data' and whilst it is of finite size, panning and zooming should allow you to present any view on it that you require.
I hope this helps!