gnuplot documentation

60 PLOT gnuplot 4.3 75
In this example the data are read from the file "MyData" using a more complicated format:
plot ’MyData’ using "%*lf%lf%*20[^\n]%lf"
The meaning of this format is:
%*lf
%lf
%*20[^\n]
%lf
ignore a number
read a double-precision number (x by default)
ignore 20 non-newline characters
read a double-precision number (y by default)
One trick is to use the ternary ?: operator to filter data:
plot ’file’ using 1:($3>10 ? $2 : 1/0)
which plots the datum in column two against that in column one provided the datum in column three
exceeds ten. 1/0 is undefined; gnuplot quietly ignores undefined points, so unsuitable points are
suppressed.
In fact, you can use a constant expression for the column number, provided it doesn’t start with an
opening parenthesis; constructs like using 0+(complicated expression) can be used. The crucial
point is that the expression is evaluated once if it doesn’t start with a left parenthesis, or once for each
data point read if it does.
If timeseries data are being used, the time can span multiple columns. The starting column should be
specified. Note that the spaces within the time must be included when calculating starting columns for
other data. E.g., if the first element on a line is a time with an embedded space, the y value should be
specified as column three.
It should be noted that plot ’file’, plot ’file’ using 1:2, and plot ’file’ using ($1):($2) can be subtly
different: 1) if file has some lines with one column and some with two, the first will invent x values
when they are missing, the second will quietly ignore the lines with one column, and the third will store
an undefined value for lines with one point (so that in a plot with lines, no line joins points across the
bad point); 2) if a line contains text at the first column, the first will abort the plot on an error, but the
second and third should quietly skip the garbage.
In fact, it is often possible to plot a file with lots of lines of garbage at the top simply by specifying
plot ’file’ using 1:2
However, if you want to leave text in your data files, it is safer to put the comment character (#) in the
first column of the text lines.
Feeble using demos.
If gnuplot is built with configuration option –enable-datastrings, then additional modifiers to using can
specify handling of text fields in the datafile. See datastrings (p. 24), using xticlabels (p. 75),
using title (p. 75).
60.2.9.1 Using title If gnuplot is built with configuration option –enable-datastrings, then the first
entry of a column of the input data file can be used as a string to provide the plot title in the key box.
The column containing specified is independent of the column[s] used for the plot itself.
plot ’data’ using 1:($2/$3) title column(N)
In this case the entry in the first row of column N will be used for the key entry of the plot constructed
from dividing column 2 by column 3. The entry in the first row of columns 2 and 3 will be ignored.
60.2.9.2 Xticlabels If gnuplot is built with configuration option –enable-datastrings, then a column
of the input data file can be used to label axis tic marks. The format of such a plot command is
plot ’datafile’ using ::xticlabels() with