fio2gnuplot: Fixing Yaxis rendering for 2D plots

With many values, gnuplot was complaining about too many ticks like :

"graph2D.gpm", line 26: warning: Too many axis ticks requested (>9e+03)

Using the "auto" command, that works much more better and axis are now
readable
diff --git a/tools/plot/graph2D.gpm b/tools/plot/graph2D.gpm
index 2d0d30a..efc7ac2 100644
--- a/tools/plot/graph2D.gpm
+++ b/tools/plot/graph2D.gpm
@@ -8,7 +8,7 @@
 
 #Preparing Axes
 #set logscale x
-set ytics axis out
+set ytics axis out auto 
 #set data style lines
 set key top left reverse
 set xlabel "Time (Seconds)"