#lang racket
(require racket/gui mrlib/snip-canvas plot)

(define ((make-current-value-renderer fn) snip event x y)
  (define overlays
    (and x y (eq? (send event get-event-type) 'motion)
         (list (vrule x #:style 'long-dash)
               (point-label (vector x (fn x)) #:anchor 'auto))))
  (send snip set-overlay-renderers overlays))

(define (make-plot-snip width height)
  (define snip  (plot-snip (function sin)
                           #:x-min 0 #:x-max (* 2 pi) #:y-min -1.5 #:y-max 1.5
                           #:width width #:height height))
  (send snip set-mouse-event-callback (make-current-value-renderer sin))
  snip)

(define toplevel (new frame% [label "Plot"] [width 500] [height 200]))
(define canvas (new snip-canvas% [parent toplevel] [make-snip make-plot-snip]))
(send toplevel show #t)