In reply to Fritz Scholz.

How to do it: Assuming that you want a line in the log-log coordinates, work entirely in the transformed coordinate system:
proc sgplot data=spirit;
scatter y = weightL10 x=TO_DistL10;
lineparm ...; /* specify point-slope in the transformed coordinates */
run;

I don't know why the LINEPARM isn't supported for TYPE=LOG. The convention is to specify points, slope, reference lines, etc., in the DATA coordinate system. But (in general) a line in the data coordinates gets mapped to a nonlinear curve in the transformed coordinates. Maybe that is why the LINEPARM statement is blocked?