Same thing as all the others, except the graph is defined by level.
I'd prolly have it done the same way the BGs are divided up (i.e. 10-19, 20-29,... 60). All on one graph (or optional to take the lines off, so we can see only 1 level versus another).
It would probably (to accurately reflect the data) be the percentage of the players which fall into the category.
On the Y-axis we would have 0%-100%, on the X-axis we would have the normal 0-24 (23?). Each level bracket would be represented by a different color line.
Not that it would be hard to do, but pretty much impossible with current data and current data collection methods. I would have to add in some additional trackers to do this. I have thought about doing exactly this but haven't included it as of yet.