A radar chart has the value 0 at the center and then extends to 1 towards the vertices of the polygon, but I'm looking for one where the value goes from 0 from the opposite edge of the vertex.
Bonus question: Is this type of chart limited to triangles?
These are called ternary plots, plotting three variables that sum to a constant, and rely on Viviani's theorem for their correctness. Here's an example.
Two dimensions only suffice for plotting a system of variables with two degrees of freedom, although an extension of Viviani's theorem means that a tetrahedron can be used to plot four variables that sum to a constant, and similarly for higher dimensions.