Support file/convertible 'png-bytes+bounds ?

[greghendershott]

I noticed that objects produced by plot don't seem to support the 'png-bytes+bounds format for file/convert's convert:

#lang racket/base

(require plot)
(define p (plot (function (λ (x) (sin (* 4 x))) -1 1)
                #:x-min -1.5 #:x-max 1.5y-min -1.5 #:y-max 1.5))

(require file/convertible)
(and (convert p 'png-bytes)        #t) ;#t Hooray!
(and (convert p 'png-bytes+bounds) #t) ;#f Boo!

Although I'm not familiar with plot, I think I managed to follow the breadcrumbs from plot to image-snip%, and to this code in this snip-lib repo:

(define png-convertible<%>
  (interface* ()
              ([prop:convertible
                (lambda (img format default)
                  (cond
                    [(or (eq? format 'png-bytes)
                         (and (eq? format 'png@2x-bytes)
                              (= 2 (send (send img get-bitmap) get-backing-scale))))
                     (let ([s (open-output-bytes)])
                       (send (send img get-bitmap) save-file s 'png
                             #:unscaled? (eq? format 'png@2x-bytes))
                       (get-output-bytes s))]
                    [else default]))])))

I considered attempting a PR, but, I have very low hands-on mileage with Racket GUI things. For example, I'm not confident how to get the bounds. Maybe using snip% get-extent ... but maybe not that simple? So for now I wanted to just open this issue in case someone else might find this easy and/or interesting to do?

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p.s. For me personally, this isn't urgent. It's barely even nice-to-have: It would enable the Racket Mode REPL pretty-print short lists of small plots on a single line. Not a huge deal IMHO. But maybe more important to other people doing other things.