Take instrumentation overhead into account when plotting.

tools/plot-timer-events

Index: tools/plot-timer-events
diff --git a/tools/plot-timer-events b/tools/plot-timer-events
index efa82bc5cda1bff133e1421f3a730165053d36bd..581e0ae333081dc421b976ca4b70d220f148980a 100755
--- a/tools/plot-timer-events
+++ b/tools/plot-timer-events
@@ -32,9 +32,40 @@ if [ ! -x "$d8_exec" ]; then
   exit 1
 fi
 
-# nm spits out 'no symbols found' messages to stderr.
-cat $log_file | $d8_exec $tools_path/csvparser.js \
+if [ -n "$DISTORTION" ]; then
+  distortion=$DISTORTION
+else
+  # Try to find out how much the instrumentation overhead is.
+  calibration_log=calibration.log
+  calibration_script="for (var i = 0; i < 1000000; i++) print();"
+
+  $d8_exec --nocrankshaft --prof --logfile $calibration_log \
+       --log-timer-events -e "$calibration_script" > /dev/null
+  t_1=`grep "V8.Execute" $calibration_log  | tail -n1 | awk -F, '{print $4}'`
+  n_1=`grep "timer-event" $calibration_log  | wc -l`
+
+  $d8_exec --nocrankshaft --prof --logfile $calibration_log \
+       --log-internal-timer-events -e "$calibration_script" > /dev/null
+  t_2=`grep "V8.Execute" $calibration_log  | tail -n1 | awk -F, '{print $4}'`
+  n_2=`grep "timer-event" $calibration_log  | wc -l`
+
+  rm $calibration_log
+
+  # Overhead in picoseconds.
+  distortion=`echo "1000*($t_1 - $t_2)/($n_1 - $n_2)" | bc`
+fi
+
+if [ -n "$PLOT_RANGE" ]; then
+  plot_range=$PLOT_RANGE
+else
+  plot_range=auto,auto
+fi
+
+echo "DISTORTION=$distortion"
+echo "PLOT_RANGE=$plot_range"
+
+echo -e "plot-range,$plot_range\ndistortion,$distortion" | cat - $log_file |
+    $d8_exec $tools_path/csvparser.js \
     $tools_path/splaytree.js $tools_path/codemap.js \
     $tools_path/logreader.js $tools_path/plot-timer-events.js \
     2>/dev/null | gnuplot > timer-events.png
-