The xerrorbars style is only relevant to 2D data plots. xerrorbars is
like points, except that a horizontal error bar is also drawn. At each point
(x,y), a line is drawn from (xlow,y) to (xhigh,y) or from (x-xdelta,y) to
(x+xdelta,y), 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.
The basic style requires either 3 or 4 columns:
3 columns: x y xdelta
4 columns: x y xlow xhigh
An additional input column (4th or 5th) may be used to provide information
such as variable point color.
