Set color to grey for points below some cutoff when plotting with palette in Gnuplot

user_2838922 0
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Sorry for the long and probably poor title.

I am trying to plot some data in gnuplot, where the data file has the format

x       y       z
1.0     1.5     0.1
1.3     1.8     0.7
2.1     3.7     1.1
3.1     4.3     1.5
3.7     4.7     1.8

I want to plot y vs. x and, for each point, indicate the value of z by the color of the point. Currently I use the following code for this:

size = 0.5
type = 7
set term pngcairo 
set output 'Foo.png'
set palette rgbformulae 22,13,-31
set cblabel "z"
unset key
set xlabel 'x'
set ylabel 'y'
set title 'y vs. x'
plot "Data.txt" u 1:2:3 w p pt type ps size palette

This works, but I would like to make the following change: For each point whose z value is below a cutoff, say 1.0, I want the color to be set to grey, regardless of the z-value. Then for z-values greater than or equal to the cutoff, I want the color to be picked from the palette according to the z-value.

Thus, I want the color to go discontinuously from grey to blue (for the current palette) at the cutoff value. Can this be done, and if so how?

gnuplot
3 Answers

It is difficult to do with a palette based on rgbformulae. It's relatively easy with a defined palette. Here is an example that gives a palette close to what you chose, with a dark gray tacked on at the bottom end. cbrange is then set so that your z cutoff is the minimum. All z values below cbmin will be assigned the endpoint value, which is dark gray in this case.

set xrange [0:1]; set yrange [0:pi]
set sample 100; set isosample 100
set palette defined (-0.01 "grey20", 0 "blue", 0.4 "green", 0.6 "yellow", 1.0 "red")

zmin = -0.6
set cbrange [zmin: *]
set xyplane at zmin
splot cos(x)*sin(y+y*(x*x)) with pm3d

enter image description here

user_67979530

After gnuplot 5.4, this can be achieved by using the function "palette()" in "using" and combining it with the "with point lc rgb variable".

size = 3 ### 0.5 in original code 
type = 7
set term pngcairo 
set output 'Foo.png'
set palette rgbformulae 22,13,-31
set cblabel "z"
unset key
set xlabel 'x'
set ylabel 'y'
set title 'y vs. x'

set cbrange [0:2]  ### this is required by palette() function
gray = 0x808080    ### gray color expressed by integer

plot NaN w p palette, \
     "Data.txt" u 1:2:($3<1 ? gray : palette($3)) w p pt type ps size lc rgb variable

The RGB value passed to "lc rgb variable" are determined by comparing $3 with threshold value 1(for example).

"NaN w p palette" is a dummy plot, a technique for forcing the colorbox to appear. Without it, the colorbox will not be displayed.

enter image description here

user_143453280

Here is a gnuplot 5.2 version. You can use a rgbformulae palette. It uses the datablock $PALETTE which is created after carrying out the command test palette.

Code: (tested with gnuplot as low as 5.2.0)

### cut-off colors for a given rgbformulae palette (gnuplot >=5.2)
reset session

# create some test data
set xrange [-5:5]
set yrange [-5:5]
set samples 50
set isosamples 50
set table $Data 
    splot (sin(1.3*x)*cos(.9*y)+cos(.8*x)*sin(1.9*y)+cos(y*.2*x))*3
unset table

set palette rgbformulae 22,13,-31
test palette   # necessary to put the palette into the datablock $PALETTE
myColorMin = 0
myColorMax = 6
myOutOfRangeColor = 0x808080
myColorPos(c) = int(255.0 * (c-myColorMin)/(myColorMax - myColorMin))+1
myColor(c) = c<myColorMin || c>myColorMax ? myOutOfRangeColor : \
             (int(word($PALETTE[myColorPos(c)],2)*0xff)<<16) + \
             (int(word($PALETTE[myColorPos(c)],3)*0xff)<<8) + \
             (int(word($PALETTE[myColorPos(c)],4)*0xff))

set cbrange [myColorMin:myColorMax]
set view 36, 72
set ztics 5

splot $Data u 1:2:3:(myColor($3)) w pm3d  lc rgb variable notitle
### end of code

Result:

enter image description here

user_72955990
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