Hello,
There is no method for modifying the line type of the grids behind the XY Graph. I would recommend making the control limit lines dashed in a different color than the grid lines, and in fact avoiding the grid line color altogether when making limit and data plots.
I tried this with LabVIEW 7.0, and I did not see the overlaid red-color plot change when it was placed over the grid line. Are you using 7.0? Can you post screen shots or VIs displaying the behavior, if you are still seeing the problem?
I hope this helps! Have a great day!
Liz Fausak