Contour breaks function works in absence of bins and binwidth (#5703)

* prioritise `breaks = function(){}`

* add news bullet

Author: teunbrand

NEWS.md

@@ -1,6 +1,9 @@
 
 # ggplot2 (development version)
 
+* When passing a function to `stat_contour(breaks)`, that function is used to
+  calculate the breaks even if `bins` and `binwidth` are missing 
+  (@teunbrand, #5686).
 * `geom_step()` now supports `lineend`, `linejoin` and `linemitre` parameters 
   (@teunbrand, #5705).
 * Fixed performance loss when the `.data` pronoun is used in `aes()` (#5730).

R/stat-contour.R

@@ -172,10 +172,8 @@ contour_breaks <- function(z_range, bins = NULL, binwidth = NULL, breaks = NULL)
   breaks_fun <- fullseq
   if (is.function(breaks)) {
     breaks_fun <- breaks
-  }
-
-  # If no parameters set, use pretty bins
-  if (is.null(bins) && is.null(binwidth)) {
+  } else if (is.null(bins) && is.null(binwidth)) {
+    # If no parameters set, use pretty bins
     breaks <- pretty(z_range, 10)
     return(breaks)
   }
@@ -206,7 +204,7 @@ contour_breaks <- function(z_range, bins = NULL, binwidth = NULL, breaks = NULL)
   }
 
   # if we haven't returned yet, compute breaks from binwidth
-  breaks_fun(z_range, binwidth)
+  breaks_fun(z_range, binwidth %||% (diff(z_range) / 10))
 }
 
 #' Compute isoband objects