Finding an X value for a given value for a grpah w/o an equation

Welcome to the forum.

Post a workbook and explain in context.

Although I would generally agree that it's better to post a workbook, I think I know what you're asking and I think I can give you a generalized solution.

A short segment of this type of curve can probably be approximated as a straight line. I would use linear interpolation. If you have an x value, find the two known data points that straddle your desired x value, (x1, y1) and (x2, y2). The corresponding y value would be

y = y1+(y2-y1)*(x-x1)/(x2-x1)

You can turn that into an Excel formula if you need to put it your workbook.

If you post your workbook we can confirm whether this is appropriate and give you a formula specific to your data.