Creating a new scale for ggplot2

gistfile1.r

# 1. functions for yearmon transformer ================ 

# Convert from numeric to yearmon
to_yearmon <- function(x)  as.yearmon(x)

# Inverse: convert from yearmon to numeric
from_yearmon <- function(x) {
  if (!inherits(x, "yearmon")) {
    stop("Invalid input: date_yearmon works with objects of class yearmon only",
      call. = FALSE)
  }
  as.numeric(x)
}

# Returns a _function_ that takes a yearmon object as input and returns a 
# yearmon object as output, with approximately n breaks, using pretty_breaks().
# Can't just use pretty_breaks() directly, since it changes yearmon objects
# to numeric.
# n is number of breaks
# (Does this really need to be a function returning a function?)
yearmon_breaks <- function(n = 5) {
  function(x) as.yearmon(pretty_breaks(n)(x))
}

# Takes a yearmon as input and returns a string representation of it
yearmon_format <- function(x) {
  paste("foo", as.numeric(x))
}

# 2. yearmon transformation object ================ 

# Returns a function that creates a transformer
# (Does this really need to be a function returning a function?)
yearmon_trans <- function() {
  require(scales)
  trans_new("yearmon", transform = from_yearmon, inverse = to_yearmon,
    breaks = yearmon_breaks(), format = yearmon_format)
}


# 3. Create the x and y scales for yearmon ================ 

# This tells ggplot2 what scale to look for, for yearmon
scale_type.yearmon <- function(x) "yearmon"

scale_yearmon <- function(aesthetics, ...) {
  continuous_scale(aesthetics, "yearmon", identity,
    guide = "none", trans = yearmon_trans(), ...)
}

scale_x_yearmon <- function(...) {
  scale_yearmon(aesthetics = c("x", "xmin", "xmax", "xend"), ...)
}

scale_y_yearmon <- function(...) {
  scale_yearmon(aesthetics = c("y", "ymin", "ymax", "yend"), ...)
}


# =====================================================================
# Usage example
# =====================================================================

library(zoo)
dates <- as.yearmon(seq(as.Date('2008-12-01'), by='month', to=as.Date('2012-06-01')))
data <- data.frame(Date=dates, value=rnorm(length(dates)))

# If you use scale_x_continuous and specify the transformer, you only need
# sections 1 and 2 from above.
ggplot(data, aes(x=Date, y=value)) + geom_line() +
  scale_x_continuous(trans = yearmon_trans())

# To have ggplot automatically figuure out the scale, you also need section 3
ggplot(data, aes(x=Date, y=value)) + geom_line()