R { ggplot2 } Is it possible to query what the tick marks are for a plot?

问题

Using the example from Hadley's website:

> (m <- qplot(rating, votes, data=subset(movies, votes > 1000), na.rm = T))

Which creates:

My Question: Is it possible to determine what the ticks marks after creating the plot object? (I want to remove the first auto-generated breakpoint)

Background: In the above plot, one can clearly see that the x-axis breaks are at 2 through 9. To obtain this manually, use:

m + scale_x_continuous( breaks = c(2:9) )

But I would like to determine, from the figure, what the tick marks are so that I can remove some of them. In other words, is there a function which will return the tick marks:

myBreaks <- tickMarks(m)

So that I can subsequently call:

m + scale_x_continuous( breaks = myBreaks[-1] )

where I've removed the first break from the array.

回答1:

I'm not sure this is what you want, but you can do a hack by:

# drop first break
sx <- scale_x_continuous()
sx$.tr$input_breaks <- function(., range) grid.pretty(range)[-1]
m <- qplot(rating, votes, data=subset(movies, votes > 1000), na.rm = T)
m + sx

# reduce the breaks into half
sx$.tr$input_breaks <- function(., range) {
  r <- grid.pretty(range); r[seq_len(length(r)/2)*2]
}
m + sx

# set the (rough) number of breaks  
sx$.tr$input_breaks <- function(., range) pretty(range, 3)
m + sx

but note that this also affects y-axis...

And probably it is the easy way to make your own transform object.

TransIdentity2 <- Trans$new("identity2", "force", "force", "force")
TransIdentity2$input_breaks <- function(., range) pretty(range, 3)

m + scale_x_continuous(trans="identity2")

in this case, it does not affect y-axis.

来源：https://stackoverflow.com/questions/6501538/r-ggplot2-is-it-possible-to-query-what-the-tick-marks-are-for-a-plot