To set the attributes for the contour lines drawn on trend-surfaces and scatterplots with data-values:

ch.Set 'CAttribs' 'red,solid,0.5' 

The mnemonics may be given in any order, or you can use numbers strictly in the sequence shown above (colour, style, weight). The line weights are ‘hair, fine, medium, broad’ or you can give an exact line weight in points. The line styles are ‘solid, dot, dash, ddash, invisible’ or you can use numbers from 1 to 5.

See also ...
ch.Cloud – 3D scatter plots