Hi,
This because there are many space white area in line plot graph, so can set BehindData.nVal to 1. But for contour plot, there is not white area, so need set BehindData.nVal to 0 to not behind of data plot, and need to set fill color as NONE.
// set border color to red and increame line width to make border clear more
tr.Root.Border.Color.nVal = SYSCOLOR_RED;
tr.Root.Border.Width.dVal = 3;
tr.Root.Fill.Color.nVal = -4; // no fill color
tr.Root.BehindData.nVal = 0; // set graph object not back of data
Iris