The yerrorlines (or errorlines) style is only relevant to 2D data
plots. yerrorlines is like linespoints, except that a vertical error line
is also drawn. At each point (x,y), a line is drawn from (x,y-ydelta) to
(x,y+ydelta) or from (x,ylow) to (x,yhigh), depending on how many data columns
are provided. The appearance of the tic mark at the ends of the bar is
controlled by set errorbars. Either 3 or 4 input columns are required.
3 columns: x y ydelta
4 columns: x y ylow yhigh
An additional input column (4th or 5th) may be used to provide information
such as variable point color.
See also
http://www.gnuplot.info/demo/mgr.htmlerrorbar demo.
