static gboolean
motion_notify_cb (GtkWidget      *widget,
                  GdkEventMotion *event,
		  gpointer        user_data)
{
  MyWidgetPrivate *private_data;
  GdkDevice *device;
  GdkPoint coords;
  gdouble x, y;

  private_data = MY_WIDGET (widget)->private;

  if (!gdk_event_get_coords (event, &x, &y))
    return FALSE;

  device = gdk_event_get_device ((GdkEvent *) event);

  coords = g_new (GdkPoint);
  coords->x = (gint) x;
  coords->y = (gint) y;

  g_hash_table_replace (private_data->device_coords,
                        device, coords);

  return TRUE;
}