add note about variable name picking as suggested by alex

git-svn-id: svn://svn.oetiker.ch/rrdtool/branches/1.2@836 a5681a0c-68f1-0310-ab6d-d61299d08faa

diff --git a/program/doc/rrdgraph_data.pod b/program/doc/rrdgraph_data.pod
index 64c559aba27f36485883ea513484487a46cdc112..6c8b28a159b2732682610af1e3d680dcdb822fae 100644 (file)
--- a/program/doc/rrdgraph_data.pod
+++ b/program/doc/rrdgraph_data.pod
@@ -22,6 +22,10 @@ own manual page.
 Variable names (I<vname>) must be made up strings of the following characters
 C<A-Z, a-z, 0-9, -,_> and a maximum length of 255 characters.
 
+When picking variable names, make sure you do not choose a name that is
+already taken by an RPN operator. A save bet it to use lowercase or
+mixedcase names for variables since operators will always be in uppercase.
+
 =head1 DEF
 
 B<DEF:>I<E<lt>vnameE<gt>>=I<E<lt>rrdfileE<gt>>:I<E<lt>ds-nameE<gt>>:I<E<lt>CFE<gt>>[:step=I<E<lt>stepE<gt>>][:start=I<E<lt>timeE<gt>>][:end=I<E<lt>timeE<gt>>][:reduce=I<E<lt>B<CF>E<gt>>]