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x axis on top of graph in gnu plot

I'm able to put an x axis on the top of a graph in gnu plot via

set x2label "label" and set x2tics

I have a column of data of data1 values and I'd like to plot these values multiplied by two on the upper x axis against data data2 in the lower x axis. Both data1 and data2 are in the same data file. Here is some sample data 

data2     data1
20        1.2e-2
40        3.0e-3
60        1.4e-3
...          ...

I'd like to plot 2*data1 on upper axis against data2 on lower axis. Preferably, I'd like to just put a tick mark on the 2*data1 axis for every data2 value. On the y axis I will plot some other quantity against data2 but all I want to ask about here is how to plot x2 versus x1.

Thanks!

Solution

  • still guessing what exactly you want. Maybe we'll find out faster with this example.

    reset session

$Data <<EOD
# x2  x1      y
20    1.2e-2  1
40    3.0e-3  2
60    1.4e-3  3
EOD

set key top center
set xtics nomirror
set x2tics nomirror

plot $Data u ($1*2):3 axes x2y1 w lp pt 7 title "y-data vs. x2-axis", \
      '' u 2:3 axes x1y1 w lp pt 7 title "y-data vs. x1-axis"

    Result:

    enter image description here

    Edit:

    You can link x1 and x2 axes via a function (in the example below conversion between nm and eV) and then set your x2tics as desired.

    1. Graph1: corresponding "odd" values from x1,
    2. Graph2: "even" values by given interval x2tics 0.2,
    3. Graph3: manual setting of x2tics.

    Example:

    ### linked axes x1, x2
reset session

set xlabel "Wavelength / nm"
set xtics nomirror
set x2label "Energy / eV"
set x2tics nomirror
set link x via 1239.8/x inverse 1239.8/x

set ylabel "Intensity / a.u."
set ytics 0.2
set samples 400
Spectrum(x) = exp(-(x-500)**2/(400))

set xrange[380:780]

set multiplot layout 3,1
    set format x2 "%.2f"
    plot Spectrum(x) w l title "Spectrum"

    set format x2 "%.1f"
    set x2tics 0.2
    replot

    set x2tics ()
    myTics = "1.7 1.8 1.9 2.0 2.1 2.3 2.5 2.7 3.0"
    do for [i=1:words(myTics)] { set x2tics add (word(myTics,i) real(word(myTics,i))) }
    replot

unset multiplot
### end of code

    Result:

    enter image description here