add color-field 2D renderer

[bdeket]

Among other things allows to make plots of fractals.
It is similar to contour-intervals, but for erratic functions (like fractals) I don't know how to get contour-intervals to display what I want.

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Sample code:

#lang typed/racket
(require plot)
;; https://rosettacode.org/wiki/Mandelbrot_set#Racket
(define (iterations [a : Number][z : Number][i : Nonnegative-Integer]) : Nonnegative-Integer
  (define z′ (+ (* z z) a))
  (if (or (<= 255 i) (< 2 (magnitude z′)))
      i
      (iterations a z′ (+ i 1))))

(define (iter->color [i : Nonnegative-Integer]) : Plot-Color
  (if (= i 255)
      'black
      (list (* 5 (modulo i 15)) (* 32 (modulo i 7)) (* 8 (modulo i 31)))))

(define (mandelbrot [x : Real][y : Real]) : Plot-Color
  (define z (make-rectangular (* 1.0 x) (* 1.0 y)))
  (iter->color (iterations z 0 0)))

(plot (color-field mandelbrot -2.25 0.75 -1.25 1.25 #:samples 200))

Produces:

[alex-hhh]

Among other things allows to make plots of fractals.

Allow me to ask the reverse question. Except for plots of fractals, what other things can color-field be used for? Also, even for fractals, is there a particular need for displaying fractals on a plot surface? You could generate fractals using pictures (e.g using the dc pict constructor).

[bdeket]

Regarding fractals:
the nice thing of doing it with plot is that zooming is there by default.

Other things that would be possible:
instead of making a vector-field, use color intensity for the length of the vector, and color for the angle
(I would not know how to do that with contours)

What I am mainly using it for:
making error plots of complex functions. Where the errors can be really erratic. and the zooming function of plot gives me the option to explore where the problems are

edit: and you get tick marks!

[alex-hhh]

This pull request looks good to me. I added some comments for adding history entries to the documentation and removing some commented out code, once you address those, I will merge the changes.