I've been banging my head againsta problem with the new Microsoft Charting control for an hour or so.
It turns out that the problem is simply that it can not be invisible!
I originally had the control in a Multiview and the View it was on was not the first view shown, you had to click a button or two and then the chart should appear by setting its view to be the active one. No luck, when it came time to display the view the "Error executing child request for ChartImg.axd" error came up.
So thinking it might be linked to the Multiview, I simply turned the chart invisible. Same problem.
But if I leave the chart on the screen at all time and simply use a style sheet to hide and show it, it works fine. So it seems that the problem is just that if the chart is first invisible ( server-side ) it will crash when you make it visible.
A bit crap really, but never mind, it's done now, phew!