RRD and negative values

[<doctor (at) makelofine.org>]

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 ?