Most terminals now allow you to set an explicit background color for the plot. The special linetype `bgnd` will draw in this color, and `bgnd` is also recognized as a color. Examples:
# This will erase a section of the canvas by writing over it in the
# background color
set term wxt background rgb "gray75"
set object 1 rectangle from x0,y0 to x1,y1 fillstyle solid fillcolor bgnd
# This will draw an "invisible" line along the x axis
plot 0 lt bgnd