Passing a parameter to a dstat plugin

I wrote a simple plugin (dstat_externalBackup.py) for dstat to monitor the filesystem free space and placed it in /usr/share/dstat and it works as intended.

€ dstat --externalBackup 3600
extern
 free 
 226G 
 226G

The code is below. As you may notice, the filesystem being monitored is hard coded in the plugin which is suboptimal. Has anyone figured out how to pass a parameter to the plugin so I can enter the filesystem to monitor on the command line?

### Author: J. Hendrix / Dag Wieers    

class dstat_plugin(dstat):
    '''
    Provides a test playground to test syntax and structure.
    '''
    def __init__(self):
        self.name = 'externalBackup'
        self.nick = ( 'free' , )
        self.vars = ( 'text' , )
        self.type = 's'
        self.width = 6
        self.scale = 0

    def extract(self):
        import subprocess
        import re
        temp1 = subprocess.check_output( ["/bin/df" , "-h" , "--output=avail", "/media/jhendrix/externalBackup" ] ) 
        temp2 = re.search( 'n([^n]+)n' , temp1 )
        self.val['text'] = temp2.group( 1 )