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I have tried GeoGebra and sketchpad but they can only draw triangles with all edges visible. What I want is, for each triangle, only the bound of the triangle strips are drawn and the overlapped area gets darker.
It should run on Windows or available online. I just want to make draft for my graph programming ideas before creating the real program so it should be free or quite cheap. I'd rather write one myself if it is too expensive.
I suspect that many drawing programs can do what you ask if you set the edges to None and the fill to translucent.
Example with InkScape (which is free & cross platform) but just about any drawing program could do this:
Set the fill to translucent: 
Set the edge to none: 
After some drawing and adding line terminators, for clarity, what I have looks like:
