circle modelisation

Hello to all of you,

I am currently in the middle of a data analysis. I started by plotting my data. I would like to know the radius of the circle passing through some points of my curves. I was thinking of drawing corresponding circles to get the radius. I tried to draw the curves directly with the smooth function but it doesn't seem to work.
Do you have any ideas that could help me please?
Do I have to use another software ?

Thank you for your help.
Here is the example of data curves used

Hi!

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