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https://en.wikipedia.org/wiki/Simplex

Common for visualizing mixtures of three things!

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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.

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2-sphere, I believe you mean to say.
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It's easy, if you normalize it so that it sums to one, it drops a dimension, and becomes an equilateral triangle.
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