GNU General Public License
MRTG is available under the terms of the GNU General Public License. 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.
One of the things I get out of MRTG is 'Being Known in the Industry'. This only works if you give credit. This means that you mention MRTG on webpages and other publications containing graphs created with MRTG. If possible, a link should be included in the publication, pointing to the MRTG homepage.