that is strange indeed. I'm not sure what the cause is right now, but I have entered into our bug tracking system, so it should be fixed in a hotfix release.
the bug number is 14299. feel free to contact Infragistics Developer Support at http://infragistics.com/gethelp to inquire about the status of this bug at any time.
I had the same problem, but setting SnapsToDevicePixels="False" on the chart solved it for me.