+ * Insert an empty square in the drawing, moving the bottom part.
+ *
+ * Note: The last point is exactly why it would not be so easy to add the
+ * vertical line functionnality as in the original version of LTT. In order
+ * to do so, we should keep all processes in the list for the duration of
+ * all the trace instead of dynamically adding and removing them when we
+ * scroll. Another possibility is to redraw all the visible area when a new
+ * process is added to the list. The second solution seems more appropriate
+ * to me.
+ *
+ *
+ * The pixmap used has the width of the physical window, but the height
+ * of the shown processes.