ggplot2's stat_contour and geom_contour throwing errors attempting to use z axis

Question

Successful Run

It's possible I do not understand the expectations, however, if I follow the example at stat_contour's documentation, I successfully get:

# Generate data
library(reshape2) # for melt
volcano3d <- melt(volcano)
names(volcano3d) <- c("x", "y", "z")

# Basic plot
v <- ggplot(volcano3d, aes(x, y, z = z))
v + stat_contour()

leading to:

Unsuccessful Run

However, when I try to do what appears to be exactly the same thing with exactly the same data types, I have errors:

my_plt <- ggplot(diamonds, aes(depth, carat, z=price))
my_plt + stat_contour()

yields:

Warning message:
Computation failed in `stat_contour()`:
(list) object cannot be coerced to type 'double' 

Debugging Attempt

The only potential difference I could imagine, especially with the error I was receiving, was an object type difference. So, here are some details there:

• volcano3d

  head(volcano3d)
  
    x y   z
  1 1 1 100
  2 2 1 101
  3 3 1 102
  4 4 1 103
  5 5 1 104
  6 6 1 105
  

  And...

  volcano3d %>% sapply(class)
       x         y         z 
  "integer" "integer" "numeric" 
  

• diamonds

  diamonds %>% select(depth, carat, price) %>% head
  Source: local data frame [6 x 3]
  
    depth carat price
    (dbl) (dbl) (dbl)
  1  61.5  0.23   326
  2  59.8  0.21   326
  3  56.9  0.23   327
  4  62.4  0.29   334
  5  63.3  0.31   335
  6  62.8  0.24   336
  

  And...

  diamonds %>% select(depth, carat, price) %>% sapply(class)
      depth     carat     price 
  "numeric" "numeric" "numeric" 
  

I cannot see any reason why I should be getting these errors. Any ideas??

Edit and Update

As per Gregor's comments below, it seems that he is correct:

• The stat_contour function can only handle regular data intervals in both the x and y dimensions.

For instance, if we look at volcano3d, the x & y dimensions of the plot are fully "regular":

volcano3d[seq(1,101,10),]

     x y   z
1    1 1 100
11  11 1 109
21  21 1 122
31  31 1 114
41  41 1 107
51  51 1 115
61  61 1 113
71  71 1 114
81  81 1  99
91   4 2 103
101 14 2 113

• in each axis, it follows a full sequence from 1,2,3,...

It's very rare that I have data in such a regular format. This makes the function fairly limited in application. Although I suppose data can be aggregated, binned and then thrown back into numerical formats in both the axes, to allow for some jerry-rigged attempt.

Oh well! I can always use geom_raster to do something nearly identical, just with a different visualization.

Thanks!