Some graphs you don't want Munin/rrd to adjust the SI units automatically. E.g., the "load" plugin sets graph_scale no because milli-load is just silly. A percentage graph is another prime example.
If this is set to "yes" units will be scaled according to the SI system. This is expected in most cases.
The config of such a plugin should say:
graph_args --upper-limit 100 -l 0 graph_scale no graph_vlabel %