What you are asking doesn't exactly make mathematical sense. There is no single mathematical way to convert a two dimensional graph into an equation, there are several ways to come up with an approximation.
If you posts the ordered pairs I can do a linear fit, or a polynomial fit, or an exponential fit or any other type of fit. You will have to specify more about the problem you are solving to determine which of these is more appropriate.
Why not post the points, along with an idea of where the data is from, and several of us here are qualified to make a fit as long as you are aware that it is just an approximation.
By the way, the program Excel (or any other similar spreadsheet program) will do any number of approximations for you.
But be warned a regression is what it is and a common error is to think a regression line says something it doesn't say at all. Again many of us would be happy to discuss your data with you if you post it.