graph positive and negative values

Hi, I'm wondering if anyone has been successful setting up and graphing an
item that fluctuates between positive and negative values?  I'm trying to
trend the temperature of my freezer.  When I created the rrd earlier today
(when positive values were generated) the graph was functional.  When I put
my sensors inside the freezer (where the temperatures drop a few degrees
below zero F) the values wouldn't drop below zero on the graph.
I tried modifying the current rrd tool with:
rrdtool tune /home/hobbit/data/rrd/tice/freezer.rrd --minimum freezerf:U
(which did drop the minimum to nan) but I still couldn't get negatives to
show on the graph.  I thought I'd delete the rrd and start over - but I
think now that the values are negative numbers the rrd won't even create.
 Here is a snippet from my hobbitgraph and hobbitserver files

[freezer]
       TITLE Freezer Temperature
       YAXIS Degrees
       DEF:tl=freezer.rrd:freezerf:AVERAGE
       #CDEF:wipeoutf=tl,-1,*
       DEF:cel=freezer.rrd:freezerc:AVERAGE
       #CDEF:wipeoutc=cel,-1,*
       LINE2:tl#@COLOR@:Temperature F
       #LINE2:wipeoutf#@COLOR@:Temperature -F
       LINE2:cel#@COLOR@:Temperature C
       #LINE2:wipeoutc#@COLOR@:Temperature -C
       COMMENT:\n
       GPRINT:tl:LAST:Temperature F\: %2.1lf (cur)
       GPRINT:tl:MAX:\: %2.1lf (max)
       GPRINT:tl:MIN:\: %2.1lf (min)
       GPRINT:tl:AVERAGE:\: %2.1lf (avg)\n
       GPRINT:cel:LAST:Temperature C\: %2.1lf (cur)
       GPRINT:cel:MAX:\: %2.1lf (max)
       GPRINT:cel:MIN:\: %2.1lf (min)
       GPRINT:cel:AVERAGE:\: %2.1lf (avg)\n


NCV_freezer="*:GAUGE"
(with freezer=ncv at the end of TEST2RRD and freezer and the end of GRAPHS).

Here is my script that generates the NCV for the rrd:

#!/bin/sh
COLUMN=freezer      # Name of the column
COLOR=green     # By default, everything is OK

[ -f ./.digitemprc ] || /usr/local/bin/digitemp_DS9097U -s/dev/ttyS0 -i

TEMPERATURE=$(/usr/local/bin/digitemp_DS9097U -s/dev/ttyS0 -a | awk '{print
$9}' | tail -1 | awk -F. '{print $1}')

if [ "${TEMPERATURE}" -gt 30 ]; then
COLOR=red
MSG="Temperature above 30 degrees"
else
COLOR=green
MSG="Temperature normal"
fi

# Section for data graphing
TEMPERATURE2_F=$(/usr/local/bin/digitemp_DS9097U -s/dev/ttyS0 -a | awk
'{print $9}' | tail -1)
TEMPERATURE2_C=$(/usr/local/bin/digitemp_DS9097U -s/dev/ttyS0 -a | awk
'{print $7}' | tail -1)

echo "freezerf : ${TEMPERATURE2_F}" > $BBTMP/freezer_f.txt
echo "freezerc : ${TEMPERATURE2_C}" > $BBTMP/freezer_c.txt
##


$BB $BBDISP "status $MACHINE.freezer $COLOR $MSG
`cat $BBTMP/freezer_f.txt`
`cat $BBTMP/freezer_c.txt`
"

rm $BBTMP/freezer_f.txt
rm $BBTMP/freezer_c.txt

exit 0


If anyone has an idea please let me know - I'd really like to trend this
data.

Thanks,

Matt