Best way to find correlation between data sets.

I have two pixel count data sets from Photoshop, both have the same intensity, but one was calculated while cached and the other was not. I am trying to find an equation that will convert the cached version into the non cached.

Things I've tried: Plotting against each other and using polyfit to find an equation, but the R^2 value was not significant.

This might be impossible because I don't know if different levels of cache were used by Photoshop. I'm looking for ideas, this is a case of I've tried nothing and I'm all out of ideas.

Thanks.