Hi!

HRULE is replaced by LINE1 because the documentation of rrdgraph once
said that HRULE was deprecated. HRULE has the interesting property of
not changing the scale of a graph and, hence, cannot be replaced with
LINE1. The deprecation warning in the documentation has been
removed. Please, remove the part of the few lines disallowing the use
of HRULE.

http://www.mail-archive.com/rrd-users@lists.oetiker.ch/msg11235.html

Thanks.