Syntax:
plot 'file' every a{:b{:c{:d}}}
This selects every a-th point in every b'th (iso)line, starting at point c in line d. To plot a single line from a 3-d datafile, one trick is to set d to the required line, and set b very large, so that no subsequent lines will be selected. If every is not specified, all points are plotted.
Following C indexing, the first point (and the first isoline) are assigned the index 0.