GNU General Public License
RRDtool is available under the terms of the GNU General Public License V2 or later. This means you can do most things you want with this software as long as you do not claim you created the software and don't sell it (or modified version of it) under a license other than the GNU GPL.
If you want to use RRDtool with an open source tool that has a GPL incompatible license, you may be able to get around the problem thanks to the FLOSS License Exception in the RRDtool license.
One of the things I get out of RRDtool is 'Being Known in the Industry'. This only works if you give credit. This means that you mention RRDtool on webpages and other publications containing graphs created with RRDtool. If possible, a link should be included in the publication, pointing to the RRDtool homepage. Best is to include one of the RRD TOOL logos on the page.