Sadly, while I wish the fitting tool would fit your data on log scales if that is how the axes are set, it does not. (That has always bugged me. But who am I to say? They made a choice that seemed valid to the author of the basic fitting tool.) It still fits your unlogged data to the desired curve.
However, you do not need to "write" the linear regression yourself. polyfit will do perfectly well.
mdl = polyfit(log(x),log(y),1)