How to color the x<0 region of the plane in Gnuplot?

Question

I am trying to color the x<0 region of a 2D plot, i.e, coloring the II and III region of the following picture

I have tried using

set style fill transparent solid 0.5 noborder
[-100:0] x<0 fs solid 1.0 lc rgb "grey" notitle

But it doesn't work, just gives a straight line instead of coloring the full halfplane. Any ideas?

Answer 1

You could place a rectangle behind the plot. Check the following:

However, I haven't found out why the rectangle gets a border, although I specified set style fill solid 1.0 noborder.

Edit: I still don't understand why gnuplot still draws a border. But I can get rid of the border when defining a fully transparent border, e.g. 0xff123456.

set obj 1 rect from graph 0,0 to first 0, graph 1 fc rgb 0xccff0000 behind 
set obj 1 fs solid 1.0 border rgb 0xff123456

Code:

### half plot with colored background
reset session
set size ratio 1

set border 0
set xrange [-5:5]
set yrange [-5:5]

set xtics 1 axis
set ytics 1 axis
set xti
set grid x,y

set label 1 "I"   at graph 0.75, graph 0.75 font ",20"
set label 2 "II"  at graph 0.25, graph 0.75 font ",20"
set label 3 "III" at graph 0.25, graph 0.25 font ",20"
set label 4 "IV"  at graph 0.75, graph 0.25 font ",20"

set obj 1 rect from graph 0,0 to first 0, graph 1 fc rgb 0xccff0000 behind 
set obj 1 fs solid 1.0 border rgb 0xff123456
set xzeroaxis ls -1
set yzeroaxis ls -1

plot sin(x)
### end of code

Result: