Fix graph when range includes negative infinity.

When a threshold or range contains negative infinity (~)
the graph failes. The routine drawing the threshold line
outputs a rrdgraph line in the form 'HRULE:~#FF0000:"...."'.
rrdtool can not the interpret ~ sign.
This fix just skips the HRULE for negative infinity.
Merge pull request #69 from fries/master

Fix graph when range includes negative infinity.