It turns out the problem is a bug in WPF exposed by using a BitmapCache CacheMode in combination with child windows. A google search returns others seeing the same issue (along with solutions that may or may not work for you).
There are a few ways to fix this problem:
* preferred: as mentioned earlier, disabled hardware rendering on the window
* untested: create the child window on a separate UI thread
* hackish: set CacheMode=null on all visual descendants of the graph (this works as long as the number of gridlines and cursors/annotations don't change, and the number of plots stay constant if you are using vector rendering)
