Fill several sections below a curve of data in Gnu

I have a set of points "data" defining a curve that I want to plot with bezier smooth. So I want to fill the area below that curve between some pairs of x values. If I only had one pair of x values it's not that difficult because I define a new set of data and plot it with filledcu. Example:

The problem is that I want to do that several times in the same plot.

Edit: Minimal working example:

#!/usr/bin/gnuplot
set terminal wxt enhanced font 'Verdana,12'

set style fill transparent solid 0.35 noborder
plot 'data' using 1:2 smooth sbezier with lines ls 1
pause -1

Where the structure of 'data' is:

x_point y_point

And I realized that my problem is that in fact I can't fill not even one curve, it seems to be filled because the slope is almost constant there.

To fill parts below a curve, you must use the filledcurves style. With the option x1 you fill the part between the curve and the x-axis.

In order to fill only parts of the curve, you must filter your data, i.e. give the x-values a value of 1/0 (invalid data point) if they are outside of the desired range, and the correct value from the data file otherwise. At the end you plot the curve itself:

set style fill transparent solid 0.35 noborder
filter(x,min,max) = (x > min && x < max) ? x : 1/0
plot 'data' using (filter($1, -1, -0.5)):2 with filledcurves x1 lt 1 notitle,\
     ''  using (filter($1, 0.2, 0.8)):2 with filledcurves x1 lt 1 notitle,\
     ''  using 1:2 with lines lw 3 lt 1 title 'curve'

This fills the range [-1:0.5] and [0.2:0.8].

To give a working example, I use the special filename +:

set samples 100
set xrange [-2:2]
f(x) = -x**2 + 4

set linetype 1 lc rgb '#A3001E'

set style fill transparent solid 0.35 noborder
filter(x,min,max) = (x > min && x < max) ? x : 1/0
plot '+' using (filter($1, -1, -0.5)):(f($1)) with filledcurves x1 lt 1 notitle,\
     ''  using (filter($1, 0.2, 0.8)):(f($1)) with filledcurves x1 lt 1 notitle,\
     ''  using 1:(f($1)) with lines lw 3 lt 1 title 'curve'

With the result (with 4.6.4):

If you must use some kind of smoothing, the filter may affect the data curve differently, depending on the filtered part. You can first write the smoothed data to a temporary file and then use this for 'normal' plotting:

set table 'data-smoothed'
plot 'data' using 1:2 smooth bezier
unset table

set style fill transparent solid 0.35 noborder
filter(x,min,max) = (x > min && x < max) ? x : 1/0
plot 'data-smoothed' using (filter($1, -1, -0.5)):2 with filledcurves x1 lt 1 notitle,\
     ''  using (filter($1, 0.2, 0.8)):2 with filledcurves x1 lt 1 notitle,\
     ''  using 1:2 with lines lw 3 lt 1 title 'curve'