r - Reverse datetime (POSIXct data) axis in ggplot

Question

I'm trying to create a plot of POSIXct times using ggplot, and would like to reverse the axis, but am struggling to make it work. I've been using scale_y_datetime, because in my real application, it's important that I have control of the breaks on this axis.

Here's an example of my problem, first with normal ordering, and then my attempt to reverse the axis.

# Some random dates and values to plot
MyData <-
  structure(list(Date = structure(c(1492979809.99827, 1492602845.68722, 
  1493093428.90318, 1492605578.0691, 1492961342.65056, 1492771976.83545, 
  1493020588.88485, 1493057018.85104, 1492852011.23873, 1492855996.55059
  ), class = c("POSIXct", "POSIXt")), Value = c(4.52885504579172, 
  6.0024610790424, 8.96430060034618, 7.06435370026156, 5.08460514713079, 
  3.47828012891114, 6.29844291834161, 0.898315710946918, 1.44857675535604, 
  5.74641009094194)), .Names = c("Date", "Value"), row.names = c(NA, 
  -10L), class = "data.frame")

library(ggplot2)
library(scales)
ggplot(MyData, aes(x=Value, y=Date)) +
  geom_point() + 
  scale_y_datetime(limits=c(min(MyData$Date),max(MyData$Date)))

which produces this:

If I attempt to reverse the Y axis, by reversing limits, I lose all breaks and data, like this:

ggplot(MyData, aes(x=Value, y=Date)) +
  geom_point() +
  scale_y_datetime(limits=c(max(MyData$Date),min(MyData$Date)))

Is there a simple way to reverse the datetime axis?

Answer 1

With the help from this post from Hadley Wickham here is how you can get a reverse datetime scale:

c_trans <- function(a, b, breaks = b$breaks, format = b$format) {
  a <- as.trans(a)
  b <- as.trans(b)

  name <- paste(a$name, b$name, sep = "-")

  trans <- function(x) a$trans(b$trans(x))
  inv <- function(x) b$inverse(a$inverse(x))

  trans_new(name, trans, inverse = inv, breaks = breaks, format=format)

}

rev_date <- c_trans("reverse", "time")

ggplot(MyData, aes(x=Value, y=Date)) +
  geom_point() + 
  scale_y_continuous(trans = rev_date)

Here is the plot: