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(gnuplot.info)User-defined

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 New user-defined variables and functions of one through five variables may
 be declared and used anywhere, including on the `plot` (Note: plot ) command
 itself.

 User-defined function syntax:
       <func-name>( <dummy1> {,<dummy2>} ... {,<dummy5>} ) = <expression>

 where <expression> is defined in terms of <dummy1> through <dummy5>.

 User-defined variable syntax:
       <variable-name> = <constant-expression>

 Examples:
       w = 2
       q = floor(tan(pi/2 - 0.1))
       f(x) = sin(w*x)
       sinc(x) = sin(pi*x)/(pi*x)
       delta(t) = (t == 0)
       ramp(t) = (t > 0) ? t : 0
       min(a,b) = (a < b) ? a : b
       comb(n,k) = n!/(k!*(n-k)!)
       len3d(x,y,z) = sqrt(x*x+y*y+z*z)
       plot f(x) = sin(x*a), a = 0.2, f(x), a = 0.4, f(x)

 Note that the variable `pi` is already defined.  But it is in no way magic;
 you may redefine it to be whatever you like.

 Valid names are the same as in most programming languages: they must begin
 with a letter, but subsequent characters may be letters, digits, "$", or "_".
 Note, however, that the `fit` (Note: fit ) mechanism uses several variables
 with names that begin "FIT_".  It is safest to avoid using such
 names.  "FIT_LIMIT", however, is one that you may wish to redefine. See the
 documentation on `fit` for details.


 See `show functions` (Note: functions ),  `show variables`
 (Note: variables ), and fit.