http://i2p-javadocs.i2p/org/rrd4j/graph/RrdGraphDef.html
Since Rrd4j outputs GIFs, PNGs,
and JPEGs it's recommended that the filename end in either .gif,
.png or .jpg. Rrd4j does not enforce this, however. If the filename is
set to '-' the image will be created only in memory (no file will be created).
