--- a/src/graphics.cc
+++ b/src/graphics.cc
@@ -40,6 +40,7 @@
 
 #include "cmd-edit.h"
 #include "defun.h"
+#include "display.h"
 #include "error.h"
 #include "graphics.h"
 #include "input.h"
@@ -98,6 +99,29 @@
   return cmap;
 }
 
+static double
+default_screendepth (void)
+{
+  return display_info::depth ();
+}
+
+static Matrix
+default_screensize (void)
+{
+  Matrix retval (1, 4, 1.0);
+
+  retval(2) = display_info::width ();
+  retval(3) = display_info::height ();
+
+  return retval;
+}
+
+static double
+default_screenpixelsperinch (void)
+{
+  return (display_info::x_dpi () + display_info::y_dpi ()) / 2;
+}
+
 static Matrix
 default_colororder (void)
 {
@@ -2013,6 +2037,44 @@
 }
 
 void
+root_figure::properties::update_units (void)
+{
+  caseless_str xunits = get_units ();
+
+  Matrix ss = default_screensize ();
+
+  double dpi = get_screenpixelsperinch ();
+
+  if (xunits.compare ("inches"))
+    {
+      ss(0) = 0;
+      ss(1) = 0;
+      ss(2) /= dpi;
+      ss(3) /= dpi;
+    }
+  else if (xunits.compare ("centimeters"))
+    {
+      ss(0) = 0;
+      ss(1) = 0;
+      ss(2) *= 2.54 / dpi;
+      ss(3) *= 2.54 / dpi;
+    }
+  else if (xunits.compare ("normalized"))
+    {
+      ss = Matrix (1, 4, 1.0);
+    }
+  else if (xunits.compare ("points"))
+    {
+      ss(0) = 0;
+      ss(1) = 0;
+      ss(2) *= 72 / dpi;
+      ss(3) *= 72 / dpi;
+    }
+
+  set_screensize (ss);
+}
+
+void
 root_figure::properties::remove_child (const graphics_handle& gh)
 {
   gh_manager::pop_figure (gh);