The curve fit commands store the fit into the next higher overlay curve for the frame; e.g., if you read data into the first curve for a frame the fit is stored in the second. Curve fits are done after you read the data and are wiped out by the next read. The fit only uses the data within the x window.
Command[ ]? SH 55929 DE GS-PT-1S ;Command[ ]? XT 10 80
Command[ ]? R SM F1 ,, !read and smooth a bit
Command[ ]? LE !exponential fit
Frame to fit[F1]? ,
[Fitting 876 points starting at 139]
Log Fit: Offset= 2.467E-07 Tau= 1.809E+01
Command[ ]? P !plot data and overlaid fit