Fixpoint margin

plot-lib/plot/private/common/draw.rkt

@@ -242,6 +242,10 @@
 (define (margin-fixpoint x-min x-max y-min y-max
                          init-left init-right init-top init-bottom
                          get-vs)
+  (define x- (+ x-min init-left))
+  (define x+ (- x-max init-right 1))
+  (define y- (+ y-min init-top))
+  (define y+ (- y-max init-bottom 1))
   (let/ec return : (Values Real Real Real Real)
     (for/fold ([left   : Real  init-left]
                [right  : Real  init-right]
@@ -250,15 +254,15 @@
               ([i  (in-range 3)])
       (match-define (list (vector #{xs : (Listof Real)} #{ys : (Listof Real)}) ...)
         (get-vs left right top bottom))
-      (define param-x-min (apply min x-min xs))
-      (define param-x-max (apply max (sub1 x-max) xs))
-      (define param-y-min (apply min y-min ys))
-      (define param-y-max (apply max (sub1 y-max) ys))
+      (define param-x-min (apply min x- xs))
+      (define param-x-max (apply max x+ xs))
+      (define param-y-min (apply min y- ys))
+      (define param-y-max (apply max y+ ys))
 
-      (define new-left (round (+ left (- x-min param-x-min))))
-      (define new-right (round (- right (- (sub1 x-max) param-x-max))))
-      (define new-top (round (+ top (- y-min param-y-min))))
-      (define new-bottom (round (- bottom (- (sub1 y-max) param-y-max))))
+      (define new-left   (round (+ left   (- x- param-x-min))))
+      (define new-right  (round (- right  (- x+ param-x-max))))
+      (define new-top    (round (+ top    (- y- param-y-min))))
+      (define new-bottom (round (- bottom (- y+ param-y-max))))
 
       ;; Not enough space?
       (define area-x-min (+ x-min new-left))

plot-lib/plot/private/plot2d/plot-area.rkt

@@ -552,7 +552,7 @@
     (define: top : Real  0)
     (define: bottom : Real  0)
     (let-values ([(left-val right-val top-val bottom-val)
-                  (margin-fixpoint 0 dc-x-size init-top-margin dc-y-size 0 0 init-top-margin 0
+                  (margin-fixpoint 0 dc-x-size 0 dc-y-size 0 0 init-top-margin 0
                                    (λ ([left : Real] [right : Real] [top : Real] [bottom : Real])
                                      (get-param-vs/set-view->dc! left right top bottom)))])
       (set! left left-val)

plot-test/plot/tests/PRs/75.rkt

@@ -0,0 +1,23 @@
+#lang racket
+(require rackunit
+         plot pict
+         racket/runtime-path
+         "../helpers.rkt")
+
+;; Tests for: https://github.com/racket/plot/pull/75
+
+(define (do-plot-margin-calc output-fn)
+  (output-fn (function values 0 1)
+             #:title (standard-fish 700 350)))
+
+(define-runtime-path pr75-data "./test-data/pr75.dat")
+
+(define pr75-test-suite
+  (test-suite
+   "PR#75: margin error"
+   (test-case "pr75-margin"
+     (check-draw-steps do-plot-margin-calc pr75-data))))
+
+(module+ test
+  (require rackunit/text-ui)
+  (run-tests pr75-test-suite))