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contour(x,y,vals,level [,checkargs] [,keyword phrases]), REAL vectors x,
  y, REAL scalar level, all with no MISSING, REAL matrix vals, optional
  LOGICAL scalar checkargs

Keywords: contour graphs
contour(x,y,vals,level) determines coordinates of a polygonal curve
(curve made up of line segments) that approximates a contour of constant
height of a surface whose height at (x[i], y[i]) is vals[i,j].

x and y must be REAL vectors of unique non-MISSING values.  vals must
be a REAL nrows(x) by nrows(y) matrix, which may have MISSING elements.
level must be a non-MISSING REAL scalar, preferably between the extreme
values in vals.  Most commonly, vals[i,j] = F(x[i],y[j]) for some
function F(x,y) of two variables.  When the surface is not defined or
is infinite at (x[i],y[j]), vals[i,j] should be MISSING.

contour(x,y,vals,levels,T) does the same, except that the arguments are
not checked for correctness.  In particular, x and y are assumed to be
increasing order, which is not ordinarily required.  This usage is
designed for use in macro contourplot() which has already checked
arguments and reordered x and y before calling contour().

The value returned is structure(x:xc, y:yc, level:level).  When level
is outside the range of vals, xc and yc are both NULL.  Otherwise xc
and yc are REAL vectors of the same length. The points (xc[k],yc[k]), k
= 1,..., nrows(xc), are either intersections of the contour with
gridlines, or (MISSING,MISSING).  When a contour consists of two or
more disjoint curves, they are separated by MISSING in both xc and yc.

contour(x,y,vals,level, graphics keyword phrases) does the same except
the result is structure(x:xc, y:yc, level:level, graphics keyword
phrases) in which all the keyword phrases in the argument are appended
to the result.

  Cmd> curve <- contour(x,y,vals,level,title:"Sample contour curve")

  Cmd> if(!isnull(curve$x)){

During execution of contour(), x, y and vals are copied to invisible
arrays __X__, __Y__ and __F__, respectively and macro _Follow() traces
out the contour, keeping status information in invisible matrix __MET__.
__X__, __Y__, __F__ and __MET__ are deleted before contour() is

See also contourplot().

Gary Oehlert 2003-01-15