See fritz.py in this dir for an usage example. 
Otherwise start with:
>>> import fritzbox
>>> help(fritzbox)

I am using the following ~/.fritzboxpyrc: 

----------------------------------------
[global]
url: http://fritz.box
username: 
password: <password>
----------------------------------------

which I read on startup by calling:
fb.readConfig("/home/mathias/.fritzboxpyrc")

Track data with rrd:
rrdtool create fritz.rrd -s 300 DS:power:GAUGE:900:0:150 RRA:AVERAGE:0.5:12:3600 RRA:AVERAGE:0.5:288:3600 RRA:AVERAGE:0.5:1:3600
rrdtool fetch fritz.rrd AVERAGE
rrdtool graph fritz.png DEF:power=fritz.rrd:power:AVERAGE 'LINE1:power#ff0000:Watt'