Choosing a specific value of intensity and plotting it against X-axis

[Amruth]

Hello,

I have a picture of a boundary layer profile on a flat plate which I got from my experiment. I have converted that to a binary image(I am not sure whether this conversion was necessary or not). I would like to choose a particular value of intensity from this and plot it against the length of the plate(X-axis) and then fit a curve (polynomial fit) through it. The Y-axis will not have any values in this case I guess. How can I do it?

Thanks.

Amruth