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From: Ingo Thies <ingo.thies@gmx.de>
Newsgroups: comp.graphics.apps.gnuplot
Subject: Image or special files do not work on y2 axis?
Date: Sat, 29 Dec 2012 15:06:23 +0100
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Dear all,

I am currently trying to trick a gradient fill below a curve by the 
following method:

f(x)=-x*x+1.
xmin=-1.;xmax=1.
ymin=0.;ymax=1.2
set xrange [xmin:xmax];set cbrange [xmin:xmax]
set y2range [0:1]#dummy yrange
#set palette defined ( 0 "blue", 3 "green", 6 "yellow", 10 "red" )
set palette file 'RGB_palettes/rainbow.gpf' u 1:2:3:4; set xrange 
[xmin:xmax];set cbrange [xmin:xmax]
#set yrange [ymin:ymax]#actual yrange using y2 axis
set yrange [0:*]
unset colorbox
unset y2tics; set ytics autofreq
plot '++' using 1:2:1 axes x1y2 with image,\
f(x) axes x1y1 notitle w filledcurves below y1=ymax lc rgb "white",\
f(x) axes x1y1 t 'f(x)' w l lc rgb "#000000"

While this works without problem for the "palette defined ..." method it 
fails for "palette file ...", with an error message "y range is 
invalid". However, if I exchange y1 and y2 axes it works. My guess: 
gnuplot can define file dummy files of the type '++' and similar 
properly only if axes x1y1 is used. Otherwise the palette specification 
fails. Did I found a bug here? Or is there a bug in the script above?

IngO