Question

Hello All you Lovely People on stackoverflow,

I am trying to plot data using gnuplot. I start by reading through a table and pulling out the data I want. I write this data to a .dat file. As of now, I’m just trying to plot it through a command line but will add the necessary code to plot it from the python script after it’s working.

My code which creates the .dat file-

#!/usr/bin/python

file = open("test_m.rdb")
table = open('table.dat', 'w+')

trash = file.readline()

trash = file.readline()

data = file.readline()
i = data.split()
flux = i[2]
observed = i[4]
table.write(flux + " " + observed,)

while 1:
    line = file.readline()
    i = line.split()
    try:
        flux = i[2]
        observed = i[4]
    except IndexError:
        break
    table.write("\n" + flux + " " + observed)
    table.close()

The command I’m attempting to use in cygwin and the error-

gnuplot plot table.dat

0.058 2
^
"table.dat", line 1: invalid command

Thank you in advance. I appreciate any suggestions you can offer.

Answer 1

you probably want:

gnuplot --persist -e 'plot "table.dat" u 1:2'

With your command, gnuplot is looking for commands to run in a file called ‘plot’ and then in a file called ‘table.dat’. ‘table.dat’ doesn’t have commands to run, it has data to be plotted. using ‘-e’ is the same thing as putting the stuff in singe quotes into a temporary file (call it temp.gp) and then doing gnuplot temp.gp. The --persist makes it so the plot stays on your screen (which you’ll want since I doubt you’re saving it to a file). To learn about how to save it to a file, inside gnuplot do: help set term and help set output and set term.

EDIT

I don’t know much about cygwin, so I don’t know what the default terminal is (or what terminals will be enabled).

A few things to try:

gnuplot -e 'plot "table.dat" u 1:2; pause -1'  #this should leave your plot open until you hit return

Put the commands in a file

#tmp.gp
set term postscript enh color
set output "tmp.ps"
plot "table.dat" u 1:2

Now run it:

gnuplot tmp.gp

and then open the postscript using whatever tool you have for viewing postscripts — I often use gv, but I don’t know what there is on cygwin.

gv tmp.ps &