-I<Deprecated. Use the B<STACK> modifiers on the other commands.>
-I<Note: the comments on stacking are still valid...>
-Repeats the last B<LINEx> or B<AREA> however it doesn't start at the
-x-axis but rather on top of the previous element. This implies that
-there needs to be something to stack on. An invisible B<LINEx> or
-B<AREA> is something you can stack on, an unknown value is not!
-
-Note: When you stack on something that was I<unknown>, the whole
-stack will be I<unknown> for that point in time. If the beginning
-is undefined, there's no way to end somewhere... If you want to
-graph this stacked variable anyway you need to make sure that the
-B<LINEx> or B<AREA> it gets stacked on is not unknown. Use a CDEF
-instruction with B<IF> and B<UN> to do so.
+Plot a tick mark (a vertical line) for each value of I<vname> that is
+non-zero and not *UNKNOWN*. The I<fraction> argument specifies the
+length of the tick mark as a fraction of the y-axis; the default value
+is 0.1 (10% of the axis). Note that the color specification is not
+optional.
+
+=item B<SHIFT> : I<vname> : I<offset>
+
+Using this command B<RRDtool> will graph the following elements
+with the specified offset. For instance, you can specify an
+offset of S<( 7*24*60*60 = ) 604800 seconds> to "look back" one
+week. Make sure to notify the viewer you did so...
+As with the other graphing elements, you can specify a number or
+a variable here.