RRD and negative values

Hi,

I wrote a test reporting voltages from my motherboard's sensors. Those values are positives and negatives (+12V, -12V). I use split NCV to get values in different RRD files, and almost everything is OK, but I don't have any value for the negatives voltages. I added a - before the numeric value, but I don't get any value in RRD file : user at host:~$ rrdtool dump ~xymon/data/rrd/host/hardware,Negative_12V_volt.rrd |head -50 <!-- Round Robin Database Dump --><rrd> <version> 0003 </version> <step> 300 </step> <!-- Seconds --> <lastupdate> 1232991020 </lastupdate> <!-- 2009-01-26 18:30:20 CET -->

    <ds>
            <name> lambda </name>
            <type> GAUGE </type>
            <minimal_heartbeat> 600 </minimal_heartbeat>
            <min> 0.0000000000e+00 </min>
            <max> NaN </max>

            <!-- PDP Status -->
            <last_ds> UNKN </last_ds>
            <value> NaN </value>
            <unknown_sec> 20 </unknown_sec>
    </ds>

Could someone told me how to work with negative values ?