http://i2p-javadocs.i2p/org/rrd4j/data/DataProcessor.html
Typical class usage: final long t1 = ...
final long t2 = ...
DataProcessor dp = new DataProcessor(t1, t2);
// DEF datasource
dp.addDatasource("x", "demo.rrd", "some_source", "AVERAGE");
// DEF datasource
dp.addDatasource("y", "demo.rrd", "some_other_source", "AVERAGE");
// CDEF datasource, z = (x + y) / 2
dp.addDatasource("z", "x,y,+,2,/");
// ACTION!