gnuplot: filledcurves with color from data - gnuplot

Question

I would like to use filledcurves with palette.
My data looks like this:
0 0 0 1 1 1 1 0 0.5
2 2 2 3 3 3 3 2 0.6
My plotting commands:
set terminal pngcairo size 800,600
set output "test.png"
set style fill solid 1.0 noborder
unset border
unset xtics
unset ytics
plot '< awk ''{print $1,$2,$9,"\n",$3,$4,$9,"\n",$5,$6,$9,"\n",$7,$8,$9,"\n",$1,$2,$9,"\n"}'' rect.txt' \
using 1:2:3 with filledcurves fillcolor palette notitle
This unfortunately only yields black squares where as I would like them to be colored. Is there a way to achieve correctly colored figures with gnuplot?
EDIT:
Using as suggested:
set terminal pngcairo size 800,600
set output "test.png"
set style fill solid 1.0 noborder
unset border
unset xtics
unset ytics
file = 'rect.txt'
lines = system(sprintf('wc -l %s | cut -d'' '' -f 1', file))
frac(x) = system(sprintf('awk ''{if (NR==%d){ print $9 }}'' %s', x, file))
rect(x) = sprintf('awk ''{if (NR==%d){ print $1,$2,$4,"\n",$5,$8,$6,"\n"}}'' %s', x, file)
plot for [i=1:lines] '< '.rect(i) using 1:2:3 with filledcurves palette frac frac(i) notitle
with the content of rect.txt:
0 0 0 1 1 1 1 0 0.5
2 2 2 3 3 3 3 2 0.6
2 2 2 1 1 1 1 2 0.2
yields the following result:

Answer 1

The filledcurves plotting style doesn't support coloring by palette with data from a file. The main issue here is, that usually, when using palette, one wants to change the color within a single area depending on some data values.
To color a single rectangle taking the color value from a fractional position of the palette, you can use palette frac <value>. This requires you to iterate over all rectangles.
And for proper use with filledcurves, you need a different data format, like
x1 y1_low y1_high
x2 x2_low y2_high
Since you are using awk anyways, you can extract some more information for the actual plotting:
set terminal pngcairo size 800,600
set output "test.png"
set style fill solid 1.0 noborder
unset border
unset xtics
unset ytics
file = 'rect.txt'
lines = system(sprintf('wc -l %s | cut -d'' '' -f 1', file))
frac(x) = system(sprintf('awk ''{if (NR==%d){ print $9 }}'' %s', x, file))
rect(x) = sprintf('awk ''{if (NR==%d){ print $1,$2,$4,"\n",$5,$8,$6,"\n"}}'' %s', x, file)
plot for [i=1:lines] '< '.rect(i) using 1:2:3 with filledcurves palette frac frac(i) notitle
With the data file
0 0 0 1 1 1 1 0 0.5
2 2 2 3 3 3 3 2 0.6
2 2 2 1 1 1 1 2 0.2
you get the output