A line which is straight in a linear plot will always be curved in a log plot...? If you want it to be straight in a log10 plot, your original fit has to be exponential 10^x.
What you can do is e.g. get a linear fit g(x) to log10(y), and then f(x) = 10.^g(x) will be your actual regression function (which is obviously exponential).
