Make the gradient in three steps.
https://gyazo.com/2c38b01f144f643c7b088aeb1fe925fc
For something that is essentially a smooth gradation without boundaries, humans often think in black and white. Related: False dichotomy. Easier than thinking in gradient conditions.
It's also easier to draw diagrams.
Three levels of gradation is a drawing technique that is relatively easy to draw and avoids "false dichotomies".
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