Ternary plots can be good for visualising systems where there are three different values that always add up to a constant value (i.e. there are only two degrees of freedom). A classic example is three-day elections (example)
Does something similar exist for systems with $D>3$ variables? For example let's say I'm trying to plot the results of an election where there are four or five parties with a significant chunk of the vote.
I'm keen to stick with 2D plots due to the difficulties that can exist with interpreting 3D plots.