[rrd-users] water usage

[Rick Dicaire]

Hi folks, I'm trying to log water usage based on once daily data
input. I've tried DS types of COUNTER and GAUGE, and the graph image
doesn't appear to reflect each days usage. Here's how I'm creating the
graph:

rrdtool graph 'water.png' --title 'Water Usage' --vertical-label 'Gallons' \
--start end-1w \
'DEF:av=/backup/mrtg-2/mrtg/rrd/water.rrd:gallons:AVERAGE' \
'DEF:mi=/backup/mrtg-2/mrtg/rrd/water.rrd:gallons:MIN' \
'DEF:ma=/backup/mrtg-2/mrtg/rrd/water.rrd:gallons:MAX' \
'DEF:la=/backup/mrtg-2/mrtg/rrd/water.rrd:gallons:LAST' \
'LINE1:av#0000FF' \
'LINE1:mi#00FF00' \
'LINE1:ma#006600' \
'LINE1:la#FF0000'

Using either of the above DS types produces a steadily upward
'stepped' graph. So it appears I need to get the difference between
the LAST input and the one previous input, and graph that. How can
this be done?

Thanks