Plotting Negative Value in rrdtool graph

Recently I was configuring an mrtg/rrd graph Where I had to display negative value as well along with others, but as per my findings mrtg/rrd graph doesn’t allow plotting Negative value. Let’s say we want to plot the temperature where we get both plus and negative values at different time of the day/night.

To do so, we have to tune the required rrd file to allow such storing.

Example: we have test.rrd file.
First get info by rrdtool

rrdtool info test.rrd |grep ds

you may see following

rrdtool info test.rrd |grep ds
ds[ds0].index = 0
ds[ds0].type = "GAUGE"
ds[ds0].minimal_heartbeat = 600
ds[ds0].min = 0.0000000000e+00
ds[ds0].max = 1.0000000000e+02
ds[ds0].last_ds = "0"
ds[ds0].value = 0.0000000000e+00
ds[ds0].unknown_sec = 0
ds[ds1].index = 1
ds[ds1].type = "GAUGE"
ds[ds1].minimal_heartbeat = 600
ds[ds1].min = 0.0000000000e+00
ds[ds1].max = 1.0000000000e+02
ds[ds1].last_ds = "0"
ds[ds1].value = 0.0000000000e+00
ds[ds1].unknown_sec = 0

now modify it by

rrdtool tune test.rrd --minimum ds0:-100

Now query rrdtool info again

rrdtool info test.rrd |grep ds
ds[ds0].index = 0
ds[ds0].type = "GAUGE"
ds[ds0].minimal_heartbeat = 600
ds[ds0].min = -1.0000000000e+02
ds[ds0].max = 1.0000000000e+02
ds[ds0].last_ds = "0"
ds[ds0].value = 0.0000000000e+00
ds[ds0].unknown_sec = 0
ds[ds1].index = 1
ds[ds1].type = "GAUGE"
ds[ds1].minimal_heartbeat = 600
ds[ds1].min = 0.0000000000e+00
ds[ds1].max = 1.0000000000e+02
ds[ds1].last_ds = "0"
ds[ds1].value = 0.0000000000e+00
ds[ds1].unknown_sec = 0

You will see the difference in ds[ds0].min value. Now we are good to go with negative plotting.

An example on how to plot negative values.

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Regard’s
Syed Jahanzaib

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