gnuplot documentation

72 gnuplot 4.3 60 PLOT
60.2.6.1 Acsplines The acsplines option approximates the data with a "natural smoothing spline".
After the data are made monotonic in x (see smooth unique (p. 72)), a curve is piecewise constructed
from segments of cubic polynomials whose coefficients are found by the weighting the data points; the
weights are taken from the third column in the data file. That default can be modified by the third
entry in the using list, e.g.,
plot ’data-file’ using 1:2:(1.0) smooth acsplines
Qualitatively, the absolute magnitude of the weights determines the number of segments used to construct
the curve. If the weights are large, the effect of each datum is large and the curve approaches that
produced by connecting consecutive points with natural cubic splines. If the weights are small, the
curve is composed of fewer segments and thus is smoother; the limiting case is the single segment
produced by a weighted linear least squares fit to all the data. The smoothing weight can be expressed
in terms of errors as a statistical weight for a point divided by a "smoothing factor" for the curve so
that (standard) errors in the file can be used as smoothing weights.
Example:
sw(x,S)=1/(x*x*S)
plot ’data_file’ using 1:2:(sw($3,100)) smooth acsplines
60.2.6.2 Bezier The bezier option approximates the data with a Bezier curve of degree n (the
number of data points) that connects the endpoints.
60.2.6.3 Csplines The csplines option connects consecutive points by natural cubic splines after
rendering the data monotonic (see smooth unique (p. 72)).
60.2.6.4 Sbezier The sbezier option first renders the data monotonic (unique) and then applies
the bezier algorithm.
60.2.6.5 Unique The unique option makes the data monotonic in x; points with the same x-value
are replaced by a single point having the average y-value. The resulting points are then connected by
straight line segments.
demos
60.2.6.6 Frequency The frequency option makes the data monotonic in x; points with the same x-
value are replaced by a single point having the summed y-values. The resulting points are then connected
by straight line segments.
60.2.7 Special-filenames
There are three filenames that have a special meaning: ’ ’, ’-’, and ’+’.
The empty filename ’ ’ tells gnuplot to re-use the previous input file in the same plot command. So to
plot two columns from the same input file:
plot ’filename’ using 1:2, ’’ using 1:3
The special filename ’+’ is a mechanism to allow the full range of using specifiers and plot styles with
in-line functions. Normally a function plot can only have a single y (or z) value associated with each
sampled point. The pseudo-file ’+’ treats the sampled points as columns 1 (plot) or 1 and 2 (splot), and
allows additional column values to be specified via a using specification, just as for a true input file.
Example:
plot ’+’ using ($1):(sin($1)):(sin($1)**2) with filledcurves