The provided graph definitions are mixing aggregation functions between RRAs and VDEFs. This leads to wrong values shown in the legend which changes depending on the selected time frame.
Example mib2.bits (in mib2-graph.properties):
This definition also defines the according variables from RRAs defined as MIN and MAX accordingly but they are unused.
I'm not sure which graph definitions are affected by this or how to find them.
More details and the initial report by Simon Sorensen can be found here: http://comments.gmane.org/gmane.network.opennms.general/51100