I seem to understand this is the check on the innodb main tablespace.
Sounds like an overflow in munin check that builds the graph, due to the type of check ("MySQL InnoDB free tablespace") I would not mind too much(There's no such critical thing like too much free space!),
apart from patching the munin check to deal with numbers > 2^32.
To double check, look at:
mysql> SHOW TABLE STATUS LIKE ''G
Look at the "Data_free: " line, it is probably bigger than 2^31.
In this case it is basically not a problem, apart from Munin not able to represent that number.
For "MySQL isam/myisam table-space usage" broken image I have no idea on how that check is built,
moreover MyISAM does not have a real tablespace.
If it makes you feel better in http://demo.munin.jp/munin2/mysql-day.html that is broken too ;)