Transparent background in gnuplot multiplot

I'd like to get a transparent background in my gnuplot figure. What I coded till now is:

#! /usr/bin/python

from numpy import *
import Gnuplot as gp
import Gnuplot.funcutils

x = (1,2,3)
y=(2,4,5)
x1 = (3,6,8)
g = gp.Gnuplot()
g("set terminal svg size 200,400")
g("set output 'filename.svg'")
g("unset xtics")
g("unset ytics")
g("set multiplot layout 3,1 title 'Title here'")
g("set object 1 rectangle from screen 0,0 to screen 1,1 fillcolor rgb '#ff0000'      behind")
#First
g("set tmargin 2")
g("set title 'Plot'")
g("unset key")
d = gp.Data(x,y,with_="lines")
g.plot(d)
#Secon
g("set title 'Plot2'")
g("unset key")
d1 = gp.Data(x1,y,with_="lines")
g.plot(d1)
#Third
g("set title 'Plot3'")
g("unset key")
d2 = gp.Data(y,x,with_="lines")
g.plot(d2)
g("unset multiplot")

I got a red screen but just the third plot. (Yeah, I have to change the rgb combo to get transparent background color. But, what's it?)

Thanks in advance, FB