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Gnuplot

In the exercises we have to plot data frequently. This can be done with the programm gnuplot
. The program can be started from a linux console window with

gnuplot

. There are also Windows variants. If you want to visualize your data with you just have to write the data of your program into a file (or on the display and then redirect).

First Steps

To make a first plot, you can start doing the following (after you have started gnuplot by command see above):

      
        plot sin(x) title "sin(x)"
        replot cos(x)
        set grid
        set logscale x
        set xlabel "x"
        set ylabel "f(x)"
        set title "Plots von sinus und cosinus"
        set xrange [-pi:pi]
        set key
        replot
        unset logscale x
        replot
      
    

Here are the most important commands. For all commands there is help in the gnuplot command line:

    
      help grid
      help logscale
    
  

Plotting into files

Shall a plot e.g. be exported into an file with eps format, you can do this as following (furthermore in the gnuplot command line):

      
        set grid
        set xlabel "x"
        set ylabel "f(x)"
        set title "Plot des sinus"
        set xrange [-3*pi:3*pi]
        set key
        plot sin(x) title "sin(x)"
        set terminal postscript enhanced color
        set out "sinus.eps"
        replot
        set terminal x11
      
    

The eps file is created in the current working directory. Instead of eps files you can plot other formats by choosing the appropriate terminal (e.g. set term png). With set term x11 change to display output again. All terminals have different options, that influence the plot. Further information with build-in command: help terminal.

Plotting of data files

Data files consist of several columns. Assume we have measured for a calculation its problem size, time and speed. Then the data file can look as follows:

      
        // Commment in data file. Gnuplot knowns about comments by setting
        // of set datafile commentschars "//".
        //
        // N     t       v
        1       0.001   1090
        10      0.02    1088
        50      0.035   1023
        100     0.12    789
        500     0.23    560
        1000    2.38    452
        10000   16.76   445
      
    

Now you can choose within gnuplot, which column should be plotted. Example:

      
        set datafile commentschar "//"
        set grid
        set xlabel "N [Byte]"
        set ylabel "t [ms]"
        set title "Zeit-N Plot"
        set key
        plot "test.dat" using 2:1 with linepoints lw 3 ps 2
        set terminal postscript enhanced color
        set out "t-N.eps"
        replot
        set terminal x11
        set title "Geschwindigkeit-N Plot"
        set ylabel "v [MFLOP/s]"
        plot "test.dat" using 3:1 with lines lw 3 ps 2
        set terminal postscript enhanced color
        set out "v-N.eps"
        replot
      
    

Here would be plotted two eps files with time-N and speed-N diagram. The axes are choosen automatically.