Error Bars in Gnuplot

Error bars need one extra data column: plot with the yerrorbars style and give three columns to using — x, y, and the error size. The script below plots five measurements with symmetric uncertainties.

The file errors.dat holds the sample number, the measured value, and its uncertainty (lines starting with # are comments and are ignored):

# sample  value  uncertainty
1  10.2  0.8
2  11.5  0.6
3  10.8  0.9
4  12.3  0.7
5  13.1  1.0

set title "Measurements with Uncertainty"
set xlabel "Sample"
set ylabel "Value"
set xrange [0:6]
set grid
plot "errors.dat" using 1:2:3 with yerrorbars pointtype 7 pointsize 1.2 title "Measured", \
     "errors.dat" using 1:2 with lines linewidth 2 title "Trend"

What each part does

The key is the three-column using 1:2:3: column 1 is x, column 2 is y, and column 3 is the y error delta. With with yerrorbars, gnuplot draws a marker at each (x, y) point plus a vertical bar from y−delta to y+delta with small caps at the ends. pointtype 7 and pointsize 1.2 style the markers. The second plot clause re-reads the same file with plain using 1:2 and with lines to connect the central values as a trend line — error bars and a line through the points are separate plot elements, combined with a comma.

If your file stores explicit lower and upper bounds instead of a delta, use four columns: using 1:2:3:4, where columns 3 and 4 are y-low and y-high. Horizontal uncertainty works symmetrically with xerrorbars (using 1:2:3 = x, y, x-delta), and xyerrorbars combines both directions with four or six columns.

Variants and going further

The yerrorlines style merges the two clauses above: it draws the connecting line and the error bars in one element, plot "errors.dat" using 1:2:3 with yerrorlines. For publication figures, set the error-bar cap width with set errorbars 2.0 (in character-width units) and consider removing the trend line — a fitted curve from a real fit belongs on measurement data more than a connect-the-dots line.

Common mistakes

• Bars all zero-length or missing: the third using column points at the wrong column — count columns in the file, remembering that # comment lines are not data lines. Also confirm you did not swap in a two-column using 1:2 for the error-bar element.
• “Wrong number of columns” or truncated plot: a four-column (low/high) file plotted with a three-column spec (or the reverse). Match the using list to the file layout: 3 columns for delta form, 4 for low/high form.

Related tutorials: Curve Fitting with fit, Plot CSV Data in Gnuplot. See also the full Gnuplot Tutorials index.