Fill colour between two smooth lines drawn by data points in gnuplot - gnuplot

Question

I have file “data.txt” containing three columns. Column 1 is for x axis. I want to draw smooth curves corresponding to data points of column 2 and 3 and then want to fill colour between these two lines.
File content is;
10 -1.3 1.1
20 -0.956 0.933
50 -0.761 0.684
80 -0.523 0.439
110 -0.227 0.20
130 -0.07 0.06
My script lines are,
plot “data.txt” u 1:2 smooth bezier w filledcurves above,\
“data.txt” u 1:3 smooth bezier w filledcurves below
But I’m not getting desired shaded plot.

Answer 1

I guess you have two challenges here:
you cannot smooth and fill at the same time
you can only fill between two columns of the same dataset or file
One possible way would be the following:
plot the data smooth bezier into a table $Smooth
N is the number of lines the combined smoothed data $Smooth
merge these smoothed data by using line 1 and 1+N/2, line 2 and 2+N/2, etc. into a dataset $Paste
for the fill you then have to use columns 1, 2 and 5
Script:
### fill area between smoothed curves
reset session
$Data <<EOD
0 0 90
10 10 50
50 20 40
80 50 60
100 30 50
EOD
set table $Smooth
set samples 20
plot $Data u 1:2 smooth bezier
plot $Data u 1:3 smooth bezier
unset table
set print $Paste
N = |$Smooth|
do for [i=1:N/2] {
print $Smooth[i].$Smooth[i+N/2]
}
set print
plot $Data u 1:2:3 w filledcurves lc rgb 0xcc0000ff ti "fill between data", \
'' u 1:2 w lp pt 7 lc "red" ti "original data", \
'' u 1:3 w lp pt 7 lc "red" notitle, \
$Smooth u 1:2 w lp pt 7 lc "green" ti "smoothed curves", \
$Paste u 1:2:5 w filledcurves lc rgb 0xccff0000 ti "fill between smoothed"
### end of script
Result:

Answer 2

For future reference, the development version of gnuplot (5.5) greatly extends the options for smoothing. For your case the obvious command works as expected. The only caveat is that for open curves (endpoints are not equal) you must use smooth sbezier rather than smooth bezier.
$DATA << EOD
10 -1.3 1.1
20 -0.956 0.933
50 -0.761 0.684
80 -0.523 0.439
110 -0.227 0.20
130 -0.07 0.06
EOD
set xrange noextend
set style fill transparent solid 0.25
plot $DATA using 1:2:3 smooth sbezier with filledcurves between, \
'' using 1:2 with lp, '' using 1:3 with lp

Answer 3

As far as I understand, above and below expect three column input, and this does not work with smooth.
But starting from this and this answer, you can try something like the following:
set xzeroaxis
set tics front
plot "data.txt" u 1:3 smooth bezier notitle w filledcurves x1 lc rgb "#b0ffff00", \
"data.txt" u 1:2 smooth bezier notitle w filledcurves x1 lc rgb "#00ffffff", \
"data.txt" u 1:3 smooth bezier lt 1, \
"data.txt" u 1:2 smooth bezier lt 2
Both filledcurves plots reach from the curve to the x-axis, the second one in white covers the first one in yellow leaving the space between the two curves in yellow.
See help lc or help linecolor for color details.
This is the result: