The merge operation using JRobin is now working.

One thing:
I'm using the following code to get the date range:

 TimeSeriesDataSource dataSource ...
 long endTime = dataSource.getEndTime();
 long startTime = dataSource.getStartTime();

But, opennms-tool/opennms-rrd-converter uses:

 long endTime = dataSource.getEndTime();
 long startTime = endTime - ONE_YEAR_IN_SECONDS;

I'm not sure which one is the correct one in order to cover all the data
inside the JRBs, because in theory, assume only one year of data is
wrong.