Rob over at http://powerpivotpro.com has a interesting post on scatter charts and PowerPivot:
At right is a list of all chart types in Excel. But not all of them are supported as PivotCharts. Try to use XY (Scatter), Bubble, or Stock as a PivotChart, and youâ€™ll get an alert saying you cannot create it with pivot data.
So, do you give up?
Nope. Remember the post where I converted a PivotTable to Cube Formulas using that button on the ribbon? This is a wonderful opportunity to use that feature in an unexpected way.
Once itâ€™s converted to formulas, you can create any chart type you want against it, no restrictions. Itâ€™s just regular old cells now.
You can read more of Robâ€™s post here:
As Colin states in one of the comments to the post, thereâ€™s multiple ways to get a scatter chart on PivotTable data. From creating formulas based on the PivotTable to inserting a blank chart and using the add series dialog to select the cells. Attached to this post is a simple example.