not so FAQ

• Legend
• Tics
• Label
• 2-D Plot
• 3-D Plot
• Polar Coordinate
• Parametric Functions
• Data File
  • 
    format

  • 
    plot several data sets

  • 
    modify values

  • 
    commands in a data file

  • 
    eliminate some points

  • 
    part of data

  • 
    UNIX commands

  • 
    Date/Time Data
• After Plotting
• Miscellaneous Stuff

Plotting Numerical Data in a Data File (No.3)

How do I use UNIX commands inside gnuplot.

With gnuplot you can modify the data
in your file at plotting , but the flexibility is limited. For
example, the data to be plotted are separated into two files – X
data are in ‘a.dat’ and Y data are in ‘b.dat’, or you want to
modify a specific data point in the file. Of course the best way is
to edit your datafile before you plot it, but sometimes UNIX
commands are helpful.

Here we use the following commands.

• sort
• paste
• sed
• awk

sort

With the sort command, you can concatenate / merge several
data files.

1.0   0.1
2.0   0.2
5.0   0.5
8.0   0.8

5.0   1.0
10.0   1.2
12.0   1.3

% sort -n file1.dat file2.dat
1.0   0.1
2.0   0.2
4.0   0.6
5.0   0.5
5.0   1.0
8.0   0.8
10.0   1.2
12.0   1.3

Two datasets are shown by symbols, then all data points are
connected by one line.

gnuplot> plot "< cat -n file1.dat file2.dat" using 1:2  w l,
>             "file1.dat" u 1:2 w p,
>             "file2.dat" u 1:2 w p

paste

Paste command puts more than two files side by side.

1.0   0.1
2.0   0.2
3.0   0.5
4.0   0.8

1.0   0.6
2.0   1.0
3.0   1.2
4.0   1.3

% paste file1.dat file2.dat
1.0   0.1       1.0   0.6
2.0   0.2       2.0   1.0
3.0   0.5       3.0   1.2
4.0   0.8       4.0   1.3

To draw figure of (X,Y) pairs when X data are in ‘file1.dat’ and Y
data are in ‘file2.dat’ :

gnuplot> plot "< paste file1.dat file2.dat" using 2:4  w lp

You can compare two files with this command. The paste
command writes Y data of ‘file1.dat’ in the second column, and those of
‘file2.dat’ are in the fourth column, therefore difference between two
data sets are expressed by $2-$4, and the ratios are by
$2/$4 .

gnuplot> plot "< paste file1.dat file2.dat" using 1:($2/$4) w points

sed

The stream editor sed is very powerful tool to edit your
text data. It is impossible to explain everything here, so I show
some useful examples. The above data file ‘file1.dat’ is used as
the example.

Shift X data of 1.0 to 1.2.

gnuplot> plot '<sed "s/^   1.0/   1.2/g" file1.dat'

Show all data points, but draw a line which does not pass through
(2.0,0.2).

gnuplot> plot 'file1.dat'                   u 1:2 with points,
>             '<sed "/^   2.0/d" file1.dat' u 1:2 with lines

Remove the third line.

gnuplot> plot '<sed "3d" file1.dat' u 1:2 with points

awk

Awk is also very powerful language with which you can
manipulate each data column in a file.

Cumulative values of Y data are plotted with a bar graph. The last
value becomes the sum of Y data.

gnuplot> plot "<awk '{x=x+$2; print $1,x}' file1.dat" with boxes

Change Y into zero when Y is less than 0.2.

gnuplot> plot  "<awk '{print $1,($2<0.1) ? 0.0 : $2}' file1.dat" with lines

Multiply Y-values by 5 when X is in the range of [1:3].

gnuplot> plot  "<awk '{print $1,($1<=3 && $1>=1) ? $2*5 : $2}' file1.dat" with lines