Is there a way that I can draw a line (straight or arc) in Stellarium? I heard about scripting; no clue how to use it.
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Generally no. But it depends on what you want the lines for.
By using the line drawing capabilities in the skycultures you can make a pseudo set of lines or curves using the constellationship.fab, costellationart.fab and constellations_boundaries.dat
There is some information in the user manual
As far as I know you can't draw arcs. Lines are pretty easy -- think of it as dot to dot.
Here's the part of the user manual that briefly describes adding lines:
The files you need to edit to create lines can be opened in Notepad, but I strongly suggest you use a coding-friendly Notepad alternative.