multiplicate -1

Hello,

Im re-learning rrdtool  so i started with a simple ping graph, now i 
think i remember i could create 2 areas for 2 ping hosts if i 
multiplicated the result with *-1 i tested to add this  in the script 
that updates the .rrd

(ping -c 3 -w 6 www.google.com | grep rtt | awk -F "/" '{ print $5*-1 }' )
and it will return "-11.87" although in the graph it comes out as a 
positive result and graphs upwards when it should graph in a negative area

Im trying to google and read since i have a faint memory of that you 
could miltiplicate the info it feteches from the .rrd when you create 
the graph.

anyone got an idea or can dropkick me to the right doc to read.

cheers
/M