The 'm' is the 'milli' prefix for the units. So, 400 m means 0.400.
By default, RRDTool uses the SI prefixes -- 2000 is shown as 2k, 0.01 is shown as 10m and so on. This isn't normally an issue except when there are no units or the thing being measured doesnt make sense in fractional parts.
The way to stop this behaviour is to not use the %s in the GPRINT (this fixes the legend), and to use the --units-exponent=0 option (this fixes the Y-axis). I don't know that it is possible to make munin do this, though. You might be able to modify the plugin to add '--units-exponent 0' to the graph_args though.