pseudocolumns

Expressions in the using clause of a plot statement can refer to additional bookkeeping values in addition to the actual data values contained in the input file. These are contained in "pseudocolumns".

      column(0)   The sequential order of each point within a data set.
                  The counter starts at 0, increments on each non-blank,
                  non-comment line, and is reset by two sequential blank
                  records. For data in non-uniform matrix format, column(0)
                  is the linear order of each matrix element.
                  The shorthand form $0 is available.
      column(-1)  This counter starts at 0, increments on a single blank line,
                  and is reset by two sequential blank lines.
                  This corresponds to the data line in array or grid data.
                  It can also be used to distinguish separate line segments
                  or polygons within a data set.
      column(-2)  Starts at 0 and increments on two sequential blank lines.
                  This is the index number of the current data set within a
                  file that contains multiple data sets.  See `index`.
      column($#)  The special symbol $# evaluates to the total number of
                  columns available, so column($#) refers to the last
                  (rightmost) field in the current input line.
                  column($# - 1) would refer to the last-but-one column, etc.