Normally on a 2D-curve x is the input and y is the output, so I still don’t get what you mean, when you write that y shall be the input volume?
Nevertheless MRatio from Melda should do the job. You can get it for free, but then without the mighty modulators, which you would need for your job. To unlock them you’d need to upgrade, what is 24€ ATM. But then you have LFO’s with free designable shapes, envelope followers, transformations shapes, etc. This should be able to solve your problem, even when I don’t understand for 100% what you mean.
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