locator equivalent in ggplot2 (for maps)

Question

Note: This question is specific for mapping but I'd like to be able to use it when I plot in a standard Cartesian coordinate system.

I love base graphics but also like ggplot2 for many things. One of my most used base functions for fine tuning a graph is locator(n) but this produces an error in ggplot2.

library(ggplot2)  county_df <- map_data('county')  #mappings of counties by state ny <- subset(county_df, region=="new york")   #subset just for NYS ny$county <- ny$subregion  ggplot(ny, aes(long, lat, group=group)) +  geom_polygon(colour='black', fill=NA) locator(1) 

Now grid.locator() as pointed out to me by Dason on talkstats.com (HERE) can return something. I just don't know how to use that something to get a map coordinate.

> grid.locator() $x [1] 286native  $y [1] 133native 

Units didn't seem to help as they are not map coordinates. Maybe I need some sort of conversion.

Thank you in advance.

EDIT: (based on DWin's response)

Dwin's got the right idea but the conversion factor is a little bit off. Help with that would be appreciated. In the example below I have a map with a red dot on it at the coordinates (x = -73 & y = 40.855). I threw Dwin's response into a function to return the coordinates. I would expect the results to be the coordinates I put in but they're not.

Ideas?

require(maps); library(ggplot2); require(grid)  county_df <- map_data('county')  #mappings of counties by state ny <- subset(county_df, region=="new york")   #subset just for NYS ny$county <- ny$subregion   NY <- ggplot(ny, aes(long, lat)) +             geom_polygon(aes(group=group), colour='black', fill=NA) +           coord_map() + geom_point(aes(-73, 40.855, colour="red")) NY    gglocator <- function(object){     require(maps); require(grid)     z <- grid.locator("npc")     y <- sapply(z, function(x) as.numeric(substring(x, 1, nchar(x))))     locatedX <- min(object$data$long) + y[1]*diff(range(object$data$long))     locatedy <- min(object$data$lat)  + y[2]*diff(range(object$data$lat))     return(c(locatedX, locatedy)) }  #click on the red dot gglocator(NY)  #I expect the results to be x = -73 & y = 40.855 

EDIT 2: (Going off of Baptise's answer)

We're there

NY <- ggplot(ny, aes(long, lat)) +             geom_polygon(aes(group=group), colour='black', fill=NA) +           coord_map() + geom_point(aes(-73, 40.855, colour="red")) +           scale_x_continuous(expand=c(0,0)) + scale_y_continuous(expand=c(0,0))   NY  x <- grid.ls()[[1]][grep("panel-", grid.ls()[[1]])] #locate the panel seekViewport(x) y <-  grid.locator("npc") y <- as.numeric(substring(y, 1, nchar(y)-3))  locatedX <- min(NY$data$long) + y[1]*diff(range(NY$data$long)) locatedy <- min(NY$data$lat) + y[2]*diff(range(NY$data$lat)) locatedX; locatedy  

UPDATE: The gglocator function of the ggmap package now contains this functionality.

Answer 1

Need to use a unit system that makes sense and save the information in the ggplot object so you can convert from "npc" units to map units:

require(maps) require(grid) NY <- ggplot(ny, aes(long, lat, group=group)) +  geom_polygon(colour='black', fill=NA)  grid.locator("npc") # clicked in middle of NY State:  #$x #[1] 0.493649231346082npc # #$y #[1] 0.556430446194226npc  range(NY$data$long) #[1] -79.76718 -71.87756  range(NY$data$lat) #[1] 40.48520 45.01157  locatedX <- min(NY$data$long) + 0.493649231346082*diff(range(NY$data$long))  locatedX #[1] -75.87247 locatedY <- min(NY$data$lat) +  0.556430446194226*diff(range(NY$data$lat)) locatedY #[1] 43.00381 

Answer 2

I get the correct result if I add scale_x_continuous(expand=c(0,0)) + scale_y_continuous(expand=c(0,0)) to the plot, and seekViewport("panel-3-4") before grid.locator()