GNUPlot Manual

28 PLOT gnuplot 4.0 39
28.1 Data-file
Discrete data contained in a file can be displayed by specifying the name of the data file (enclosed in
single or double quotes) on the plot command line.
Syntax:
plot ’’ {index }
{every }
{thru }
{using }
{smooth }
The modifiers index, every, thru, using, and smooth are discussed separately. In brief, index selects
which data sets in a multi-data-set file are to be plotted, every specifies which points within a single
data set are to be plotted, using determines how the columns within a single record are to be interpreted
(thru is a special case of using), and smooth allows for simple interpolation and approximation. (splot
has a similar syntax, but does not support the smooth and thru options.)
Data files should contain at least one data point per record (using can select one data point from the
record). Records beginning with # (and also with ! on VMS) will be treated as comments and ignored.
Each data point represents an (x,y) pair. For plots with error bars or error bars with lines (see set style
errorbars (p. 98) or set style errorlines (p. 99)), each data point is (x,y,ydelta), (x,y,ylow,yhigh),
(x,y,xdelta), (x,y,xlow,xhigh), or (x,y,xlow,xhigh,ylow,yhigh).
In all cases, the numbers of each record of a data file must be separated by white space (one or more
blanks or tabs) unless a format specifier is provided by the using option. This white space divides
each record into columns. However, whitespace inside a pair of double quotes is ignored when counting
columns, so the following datafile line has three columns:
1.0 "second column" 3.0
Data may be written in exponential format with the exponent preceded by the letter e, E, d, D, q, or Q.
Only one column (the y value) need be provided.
starting at 0.
If x is omitted, gnuplot provides integer values
In datafiles, blank records (records with no characters other than blanks and a newline and/or carriage
return) are significant — pairs of blank records separate indexes (see plot datafile index (p. 40)).
Data separated by double blank records are treated as if they were in separate data files.
Single blank records designate discontinuities in a plot; no line will join points separated by a blank
records (if they are plotted with a line style).
If autoscaling has been enabled (set autoscale), the axes are automatically extended to include all
datapoints, with a whole number of tic marks if tics are being drawn. This has two consequences: i) For
splot, the corner of the surface may not coincide with the corner of the base. In this case, no vertical
line is drawn. ii) When plotting data with the same x range on a dual-axis graph, the x coordinates may
not coincide if the x2tics are not being drawn. This is because the x axis has been autoextended to a
whole number of tics, but the x2 axis has not. The following example illustrates the problem:
reset; plot ’-’, ’-’ axes x2y1
1 1
19 19
e
1 1
19 19
e
To avoid this, you can use the fixmin/fixmax feature of the set autoscale command, which turns off
the automatic extension of the axis range upto the next tic mark.
28.1.1 Every
The every keyword allows a periodic sampling of a data set to be plotted.