graph: Fix calling rrd_graph()

graph: Fix calling rrd_graph()

There is pretty much no documenation of librrd and the API has been
changed multiple times. Initially, various arguments have been labeled
optional, but that does not seem to be true for at least the
implementation I am testing against.

So we pass all variables that we function has, and we might as well
include them in the debugging output.

Signed-off-by: Michael Tremer <michael.tremer@ipfire.org>