[WIP] Add function to draw lines for treatment designs

[Copilot]

Thanks for asking me to work on this. I will get started on it and keep this PR's description up to date as I form a plan and make progress.

i want to add a function that draws two lines for altering trestment designs. The idea is, that you can define a new variabel in an scdf that contains treatment information for each mt. These are the turned into a separate line in in the scplot.

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