[B<-W>|B<--watermark> I<string>]
-Adds the given string as a watermark, horizontally centred, at the bottom
+Adds the given string as a watermark, horizontally centered, at the bottom
of the graph.
=head2 Data and variables
=head1 DESCRIPTION
If you have ever used a traditional HP calculator you already know
-B<RPN>. The idea behind B<RPN> is that you have a stack and push
+B<RPN> (Reverse Polish Notation).
+The idea behind B<RPN> is that you have a stack and push
your data onto this stack. Whenever you execute an operation, it
takes as many elements from the stack as needed. Pushing is done
implicitly, so whenever you specify a number or a variable, it gets