improvements to the color selection and pen config

By adding a new curve, the default pen is configured with width 2, symbol size 5. This is very personal but I think the default could be a bit larger.

The visual appearance may also get worsened by the fact that the color selection only optimizes for sufficient contrast between curves but not between curves and the background. I think an angular gap of 10-20% (mean rgb color) around the background color could help.