NaN in rrd file:HELP!

[CCL54]

Hello,

I'm beginner in using raspberry and cacti.
I try to get temperature measure from 1 wire ds 18B20. I've made a script that work perfectly when I call it with sudo command. I want to use cacti to get graph of temperatures but there is no data on the graph. I've followed the following tutorial : http://blog.jartweb.net/?p=999. But I still have no data in graph. I know that the scripts is called from Cacti because, in the script, I toggle an output. I also know that the CMD.php work well because in cacti log file there is the sentence: CMDPHP: poller{0}host[1]DS[8] CHD: /usr/share/cacti/site/scripts/new_temperatures.py output: T1:21.
In this file I also know that RRD is updated but i still have DS[T1].value=NaN.

I spent a lot of time on this problem but I don't know enough Cacti and rrdtool to solve the problem.

Is there someone to help me?

Thanks a lot.