2PLOT: plot label - label a plot axis

Usage: plot label <axis> <position> [<text>]

labels the given axis by placing a tic and the text at the given
position. The parameter axis can have the following values:
  x or bottom   Label the x-axis, i.e. the horizontal line at
                y-position Y0.
  y or left     Label the y-axis, i.e. the vertical line at
                x-position X0.
  top           Label the horizontal line at y-position Y1.
  right         Label the vertical line at x-position X1.
If text is blank, only a tic is set.
