Create Plots using rrdtool

Next step for my humidity logger is to get nice plots. Here rrdtool seems to be the tool of choice.

First create a few rrd-files. For example:

rrdtool create humidity.rrd --step 30 \
  DS:humidity:GAUGE:120:0:100 \
  RRA:AVERAGE:0.5:1:1200 \
  RRA:MIN:0.5:12:2400 \
  RRA:MAX:0.5:12:2400 \

With update every 30 seconds, a GAUGE value from 0 to 100. Additional calculating average, minimum and maximum of the humidity value.

Adding new values for the plot in python using python-rrdtool.

def log_to_rrd(data):
  values = json.loads(data)
  dt = time.mktime(

  ret = rrdtool.update('humidity.rrd','%d:%f' %
                       (dt, values.get('humidity')))
  if ret:
      print rrdtool.error()

And build a plot for one day:

def graph():
    ret = rrdtool.graph( fn, "--start", "-1d",
                         "--vertical-label=-" + str(cnt) + str(size),
                         "GPRINT:hum:AVERAGE:Avg Humidity\: %2.2lf%S%%",
                         "COMMENT:    ",
                         "GPRINT:hum:MAX:Max Humidity\: %2.2lf%S%%\\r")

Final example (with humidity, temperature and calculated dew point):

rrdtool example