To generate any required sequence of repeating patterns:

ch.DefineDash 12(12 4 24 4)          © Long, longer
ch.DefineDash 18(15 3 2 2 2 5)       © Long, short short
ch.DefineDash 123(3 2 3 2 3 2 12 3)  © 3xshort, then long

The argument requires a 2-element vector, giving an integer id for the pattern (use numbers above 10 to avoid clashing with patterns preset by RainPro) and the repeat pattern (on/off) in points. The repeat pattern must be of even length. You can select the pattern as for any of the RainPro presets:

ch.Set'nomark'
ch.Set'line' 1 12 18 123

This establishes the cycle of dashed lines which will be used as data series are plotted.