Thank you ZachSaucier and Greensock for your responses.
What I needed was exactly what you said Greensock, mapping a single x to y value on a custom bezier curve. And indeed CustomEase was what I needed.
So this is how I will be mapping my values:
const myEase = CustomEase.create("myEase", "M0,0 C0,0 0.056,0.442 0.175,0.442 0.294,0.442 0.332,-0.005 0.332,-0.005 0.332,-0.005 0.414,1 0.671,1 0.991,1 1,-0.005 1,-0.005");
const x = .5;
const y = myEase(x);
Many thanks again ?