memleak: Do not allow swap range (y2) to go under 0.

memleak/memory-consumption-over-time.gp

@@ -77,6 +77,8 @@ set terminal svg enhanced background rgb 'white' size mysize[1],mysize[2] \
     dynamic font myfont
 set output sprintf("%s memory consumption over time.svg", myprog)
 
+set y2range [0:]              # Don't go into minus if Swap is 0 the whole time.
+
 plot infile every 4::0 using ((timecolumn(2) - mystarttime)):($3/1024) title 'USS' \
      with linespoints linewidth 3 pointtype 7 pointsize 0.5, \
      \