Separate size calculations from drawing the plot legend (#71, #49)

The code which draws the plot legend used to calculate the legend dimensions
and draw the legend in a single method of the `plot-device%` class.  This has
been separated in two: a method to calculate the legend dimensions and one to
actually draw the legend.

This is done in order to be able to place the plot legend outside the plot
area, as part of #49

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

@@ -610,23 +610,19 @@
     ;; ===============================================================================================
     ;; Legend
 
-    (define/public (draw-legend legend-entries rect)
+    ;; the folowing functions take a (Listof legend-entry) and a Rect as argument.
+    ;; the understanding is that Rect will be the complete dc for a legend outside the plot-area
+    ;; and the plot-area otherwise
+
+    (: calculate-legend-parameters (-> (Listof legend-entry) Rect
+                                       (Values Rect Exact-Rational Exact-Rational Exact-Rational
+                                               Nonnegative-Exact-Rational Real Real
+                                               Nonnegative-Exact-Rational Real)))
+    (define/private (calculate-legend-parameters [legend-entries : (Listof legend-entry)][rect : Rect])
       (define n (length legend-entries))
-      (match-define (list (legend-entry #{labels : (Listof (U String pict))}
-                                        #{draw-procs : (Listof Legend-Draw-Proc)})
-                          ...)
-        legend-entries)
-
+      (define labels (map legend-entry-label legend-entries))
       (match-define (vector (ivl x-min x-max) (ivl y-min y-max)) rect)
 
-      (define old-size (send (send dc get-font) get-point-size))
-      (define old-face (send (send dc get-font) get-face))
-      (define old-family (send (send dc get-font) get-family))
-      (set-font-attribs
-       (or (plot-legend-font-size) old-size)
-       (or (plot-legend-font-face) old-face)
-       (or (plot-legend-font-family) old-family))
-
       (define-values (max-label-width max-label-height)
         (for/fold ([width : Exact-Rational 0]
                    [max-height : Exact-Rational 0])
@@ -679,6 +675,50 @@
       (define label-x-min (+ legend-x-min horiz-gap))
       (define draw-x-min (+ legend-x-min (* 2 horiz-gap) labels-x-size horiz-gap))
 
+      (values legend-rect top-gap baseline-skip max-label-height
+              draw-x-size label-x-min draw-x-min
+              draw-y-size legend-y-min))
+
+    (define/public (calculate-legend-rect legend-entries rect)
+      ;; Change font for correct size calculation in calculate-legend-parameters
+      (define old-size (send (send dc get-font) get-point-size))
+      (define old-face (send (send dc get-font) get-face))
+      (define old-family (send (send dc get-font) get-family))
+      (set-font-attribs
+       (or (plot-legend-font-size) old-size)
+       (or (plot-legend-font-face) old-face)
+       (or (plot-legend-font-family) old-family))
+
+      (define-values (legend-rect top-gap baseline-skip max-label-height
+                                  draw-x-size label-x-min draw-x-min
+                                  draw-y-size legend-y-min)
+        (calculate-legend-parameters legend-entries rect))
+
+      ;; Undo change font
+      (set-font-attribs old-size old-face old-family)
+
+      legend-rect)
+
+    (define/public (draw-legend legend-entries rect)
+      (match-define (list (legend-entry #{labels : (Listof (U String pict))}
+                                        #{draw-procs : (Listof Legend-Draw-Proc)})
+                          ...)
+        legend-entries)
+
+      ;; Change font early for correct size calculation in calculate-legend-parameters
+      (define old-size (send (send dc get-font) get-point-size))
+      (define old-face (send (send dc get-font) get-face))
+      (define old-family (send (send dc get-font) get-family))
+      (set-font-attribs
+       (or (plot-legend-font-size) old-size)
+       (or (plot-legend-font-face) old-face)
+       (or (plot-legend-font-family) old-family))
+
+      (define-values (legend-rect top-gap baseline-skip max-label-height
+                                  draw-x-size label-x-min draw-x-min
+                                  draw-y-size legend-y-min)
+        (calculate-legend-parameters legend-entries rect))
+
       ;; legend background
       (set-pen (plot-foreground) 1 'transparent)
       (set-brush (plot-background) 'solid)

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

@@ -133,4 +133,5 @@
    [draw-arrow-glyph (-> (Vectorof Real) Real Real Void)]
    [draw-glyphs (-> (Listof (Vectorof Real)) Point-Sym Nonnegative-Real Void)]
    [draw-pict (->* [pict (Vectorof Real)] (Anchor Real) Void)]
+   [calculate-legend-rect (-> (Listof legend-entry) Rect Rect)]
    [draw-legend (-> (Listof legend-entry) Rect Void)]))