Fixed erroneous raised exception when plotting outside the bounds of

a 2D contour plot

Please merge to 5.3.2

collects/plot/plot2d/contour.rkt

@@ -62,6 +62,9 @@
     (define num (animated-samples samples))
     (define sample (g (vector x-ivl y-ivl) (vector num num)))
     (match-define (2d-sample xs ys zss z-min z-max) sample)
+    
+    (unless (and z-min z-max) (return empty))
+    
     (match-define (list (tick zs _ labels) ...) (contour-ticks (plot-z-ticks) z-min z-max levels #f))
     
     ;; need to check this or in-cycle below does an infinite loop (I think it's an in-cycle bug)
@@ -120,6 +123,9 @@
     (define num (animated-samples samples))
     (define sample (g (vector x-ivl y-ivl) (vector num num)))
     (match-define (2d-sample xs ys zss z-min z-max) sample)
+    
+    (unless (and z-min z-max) (return empty))
+    
     (match-define (list (tick zs _ labels) ...) (contour-ticks (plot-z-ticks) z-min z-max levels #t))
     
     (define-values (z-ivls ivl-labels)

collects/plot/tests/plot2d-tests.rkt

@@ -308,6 +308,10 @@
                             #:widths '(2) #:styles '(dot)))
             #:x-min -5 #:x-max 5 #:y-min -5 #:y-max 5))
 
+;; Both plots should be blank
+(plot (contours f2 -5 0) #:x-min 1 #:x-max 3 #:y-min 1 #:y-max 3)
+(plot (contour-intervals f2 -5 0) #:x-min 1 #:x-max 3 #:y-min 1 #:y-max 3)
+
 (time (plot (contour-intervals f1 -5 5 -5 5 #:label "z")))
 
 (time (plot (contour-intervals