In the default situation, gnuplot expects to see three, four, or six numbers on each line of the data file—either (x, y, ydelta), (x, y, ylow, yhigh), (x, y, xdelta), (x, y, xlow, xhigh), (x, y, xdelta, ydelta), or (x, y, xlow, xhigh, ylow, yhigh). The x co-ordinate must be specified. The order of the numbers must be exactly as given above, though the using qualifier can manipulate the order and provide values for missing columns. For example,
plot 'file' with errorbars plot 'file' using 1:2:(sqrt($1)) with xerrorbars plot 'file' using 1:2:($1-$3):($1+$3):4:5 with xyerrorbars
The last plot is for a file with an unsupported combination of relative x and absolute y errors. The using spec generates absolute x min and max from the relative error.
The y error bar is a vertical line plotted from (x, ylow) to (x, yhigh). If ydelta is specified instead of ylow and yhigh, ylow = y - ydelta and yhigh = y + ydelta are derived. If there are only two numbers on the line, yhigh and ylow are both set to y. The x error bar is a horizontal line computed in the same fashion. To get lines plotted between the data points, plot the data file twice, once with errorbars and once with lines.
The error bar has crossbars at top and bottom unless set bar small is used.
If autoscaling is on, the ranges will be adjusted to fit the error bars.
See plot using, plot with, and set style for more information.