20 - Plotting a line paralle to y axis

If you want to plot your file like 'file.dat' then you want to have a parallel line to "y" axis, you can use:

> set arrow from 0, graph 0 to 0, graph 1 nohead
> plot 'file.dat'

In this case a line, x=0, will be plotted in your figure.
It is clear that for drwing, for example, x=1 line we need to use:

> set arrow from 1, graph 0 to 1, graph 1 nohead
> plot 'file.dat'