It is end 2013 now and I agree to Axelsson that the webmin disk usage monitor function is still very poor. A very simple tip for a simple but good
disk usage and most important "free space" overview for all partitions.
In webmin choose: others => custom commands
create a custom command, and select the following:
-description: (choose a suitable name. example: "Disk usage:")
-for the command enter: df -h
-run as user: root
That's all. No other selections needed.
Now save your command.