geom_labeled_contour

library(ggplot2)
library(dplyr)

# I guess this works only with >1.0.1
packageVersion("ggplot2")

## [1] '1.0.1.9003'

Define Stat/Geom

StatContourLabel <- ggproto(
  "StatContourLabel",
  Stat,
  default_aes = aes(label = ..level..),
  compute_group = function(..., digits = 1) {
    StatContour$compute_group(...) %>%
      group_by(level) %>%
      summarise(x = nth(x, round(n() / 2)),
                y = nth(y, round(n() / 2))) %>%
      mutate(level = signif(level, digits = digits)) 
  }
)

geom_contour_label <- function(mapping = NULL, data = NULL,
                         position = "identity", na.rm = FALSE, show.legend = NA,
                         inherit.aes = TRUE, digits = 1, ...) {
  layer(
    data = data,
    mapping = mapping,
    stat = StatContourLabel,
    geom = "text",
    position = position,
    show.legend = show.legend,
    inherit.aes = inherit.aes,
    params = list(
      na.rm  = na.rm,
      digits = digits,
      ...
    )
  )
}

geom_labeled_contour <-
  function(mapping = NULL, data = NULL, stat = "contour", position = "identity",
           lineend = "butt", linejoin = "round", linemitre = 1, na.rm = FALSE,
           show.legend = NA, inherit.aes = TRUE, digits = 1, ...) {
    list(
      geom_contour(
        mapping, data, stat, position,
        lineend, linejoin, linemitre, na.rm,
        show.legend, inherit.aes, ...
      ),
      geom_contour_label(
        mapping, data,
        position, na.rm, show.legend,
        inherit.aes, digits, ...
      )
    )
  }

Draw A Chart

N <- 60
x <- seq(0, 100, length=N)
d.cont <- data.frame(x=rep(x,N), y=rep(x,each=N))
d.cont <- transform(d.cont,
                    z.diff=x-y,
                    z.ratio=ifelse(x+y==0, 0.5, x/(x+y))
)

ggplot(d.cont, aes(x, y, z = z.diff)) +
  geom_contour() +
  geom_contour_label()

ggplot(d.cont, aes(x, y, z = z.diff / 1250)) +
  geom_labeled_contour()