Show velocity from simulator trace file

Add a tool (tools/velocity_plot.sh) to read in a simulator trace
file, calculate the velocity of the X and Y axes, and show a plot
of these velocities against time along with the X and Y positions.

tools/deriv.awk

@@ -0,0 +1,35 @@
+#!/usr/bin/awk -f
+BEGIN { firstline = 1 }
+
+# Calculates the first derivative of columns 2 and 3 wrt column 1
+$0 !~ /^#/ {
+  t = $1
+  x = $2
+  y = $3
+  if (firstline == 1) {
+    old_tx = old_ty = t
+    old_x = x
+    old_y = y
+    dy_dt = dx_dt = 0
+    firstline = 0
+  } else {
+    dx = old_x - x
+    dy = old_y - y
+
+    if ( dx != 0) {
+      dt = old_tx - t
+      dx_dt = dx/dt
+      old_tx = t
+      old_x = x
+    }
+    if ( dy != 0) {
+      dt = old_ty - t
+      dy_dt = dy/dt
+      old_ty = t
+      old_y = y
+    }
+    if ( dx != 0 || dy != 0 ) {
+      print t, " ", x, " ", y, " ", dx_dt, " ", dy_dt
+    }
+  }
+}

tools/velocity_plot.sh

@@ -0,0 +1,30 @@
+#!/bin/sh
+
+TMPDIR=/tmp
+TOOLSDIR=$(dirname $0)
+
+# Set this to your steps-per-mm value, or use 1 to keep results in steps
+MM_PER_STEP=500.0
+
+die() {
+	echo "$1"
+	exit 1
+}
+
+test -n "$1" || die "Usage: $0 <tracefile-name>"
+test -f "$1" || die "Error: $1 does not exist or is not a regular file"
+
+FILE="$1"
+VELOC="$TMPDIR/$(basename "$FILE").velocity"
+
+"${TOOLSDIR}/deriv.awk" "$FILE" > "$VELOC"
+
+gnuplot --persist -e "
+  set ylabel 'Position';
+  set y2label 'Velocity (mm/minute)'; set ytics nomirror ; set y2tics;
+  set samples 10000 ;
+
+  plot '${VELOC}' u (\$1/1000000):(\$2/$MM_PER_STEP) with lines t \"X-position\" , '' u (\$1/1000000):(\$3/$MM_PER_STEP) with lines t \"Y-position\"
+     , '' u (\$1/1000000):(60000000.0*\$4/$MM_PER_STEP) with lines t \"X-velocity\" axes x1y2
+     , '' u (\$1/1000000):(60000000.0*\$5/$MM_PER_STEP) with lines t \"Y-velocity\" axes x1y2
+"