Gnuplot is correctly fitting your data to the function you provided--a straight line.
The problem is that using a log scale for the y axis does not scale the data--just how the data are plotted.
Try fitting it to a power law:
f(x)= a*x**b
fit f(x) "d0.dat" via a,b
set logscale x
set logscale y
plot "d0.dat" with points lt rgb "#ff0000" title "Points", \
f(x) with lines lt rgb "#ff00ff" title "Approximation"