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(gnuplot.info)contour

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 `set contour` enables contour drawing for surfaces. This option is available
 for `splot` (Note: splot ) only.

 Syntax:
       set contour {base | surface | both}
       set nocontour
       show contour

 The three options specify where to draw the contours: `base` draws the
 contours on the grid base where the x/ytics are placed, `surface`
 (Note: surface ) draws the contours on the surfaces themselves, and `both`
 draws the contours on both the base and the surface.  If no option is
 provided, the default is `base`.

 See also `set cntrparam` (Note: cntrparam ) for the parameters that affect
 the drawing of contours, and `set clabel` (Note: clabel ) for control of
 labelling of the contours.

 The surface can be switched off (see `set surface`), giving a contour-only
 graph.  Though it is possible to use `set size` (Note: size ) to enlarge the
 plot to fill the screen, more control over the output format can be obtained
 by writing the contour information to a file, and rereading it as a 2-d
 datafile plot:

       set nosurface
       set contour
       set cntrparam ...
       set term table
       set out 'filename'
       splot ...
       set out
       # contour info now in filename
       set term <whatever>
       plot 'filename'

 In order to draw contours, the data should be organized as "grid data".  In
 such a file all the points for a single y-isoline are listed, then all the
 points for the next y-isoline, and so on.  A single blank line (a line
 containing no characters other than blank spaces and a carriage return and/or
 a line feed) separates one y-isoline from the next.  See also `splot datafile`
 (Note: data-file ).

 If contours are desired from non-grid data, `set dgrid3d` (Note: dgrid3d )
 can be used to create an appropriate grid.  See `set dgrid3d` for more
 information.