Custom Curve Fit

I'm relatively new to using Igor, and this is my first attempt to try to use the curve fitting feature.

I have several sets of concentration and depth data that I would like to plot all at the same time, along with curve fits that match the equation:
y = C1 - (C1-C0)*exp(-B*x)

Where C1 and B are variable, and C0 can be held (typically at 40)

Since I'm looking at change in concentration with depth, I use 'depth' as the x variable, and concentration as the y, but I swap the axes so that depth goes from 0-15 down the vertical axis, and that concentration is on the horizontal axis.

I have figured out how to take one set of this data, and plot it with the correct curve fit... however, when I try to amend another set to the graph, it shifts the curve fit down the vertical axis, and so it doesn't line up with the data anymore. for every extra trace added to the graph, the curve fit continues to get shifted.

Why does it do that? How do I stop it from doing that?

the pxp file is available here:
https://www.yousendit.com/download/YWhOZFhqY1NBNkUwTVE9PQ