http://i2p-javadocs.i2p/org/rrd4j/graph/RrdGraphDef.html
However
you may wish to display units always in k (Kilo, 10e3) even if
the data is in the M (Mega, 10e6) range for instance. Value should
be an integer which is a multiple of 3 between -18 and 18, inclusive.
It is the exponent on the units you which to use.