set title "Fill area between two curves (above/below)"
set style fill solid 1.0 noborder
set style data lines
set xrange [250:500]
plot 'silver.dat' u 1:2:($3+$1/50.) w filledcurves above title 'Above', \
               '' u 1:2:($3+$1/50.) w filledcurves below title 'Below', \
	       '' u 1:2 lt -1 lw 2 title 'curve 1', \
	       '' u 1:($3+$1/50.) lt 3 lw 2 title 'curve 2'