Every

The every keyword has special meaning when used with matrix data. Rather than applying to blocks of single points, it applies to rows and column values. Syntax:
     plot 'file' every {<column_incr>}
                         {:{<row_incr>}
                           {:{<start_column>}
                             {:{<start_row>}
                               {:{<end_column>}
                                 {:<end_row>}}}}}

Examples:
     plot 'file' matrix every :::N::N     # plot all values in row N of matrix
     plot 'file' matrix every ::3::7:     # plot columns 3 to 7 of all rows
     plot 'file' matrix every ::3:2:7:4   # submatrix of columns 3-7 rows 2-4