While you are technically correct, since any triangle is a simplex, this is not relevant to this visualization. For this visualization: get positive quantities

While you are technically correct, since any triangle is a simplex, this is not relevant to this visualization. For this visualization: get positive quantities in 3D space, normalize to 1, now you have dot on a triangle on 1-sphere in a positive octant. Project triangle into 2D space a this is your visualization.