如何获得R图中选定点的坐标？

Question

I need to pass to R program coordinates of a point selected by the mouse pointer, to perform some calculations. I have problems getting it to work.

I know that this code should identify point on a plot:

plot(kk2$k2,kk2$k1)
identify(kk2$k2,kk2$k1)

But even that doesn't work. On a plot appears some meaningless number, while point has two coordinates. why?

How to fix at least that?

My goal is to have the point coordinates returned to R and perform some calculations on them. The dataset kk2 has only two columns - k1 and k2, nothing more.

Answer 1

The package "gatepoints" available on CRAN will allow you to draw a gate returning your points of interest.

If you are using RStudio it is better to plot in a separate x11 window by first opening a new x11 device:

X11()

Now plot your points, I've made up some simple data:

kk2 <- data.frame(k2=1:10, k1=1:10)
plot(kk2, col = "red", pch = 16)

Run the command below and then select your points by left clicking and right clicking to close the polygon:

selectedPoints <- fhs(kk2)

This will return:

selectedPoints
#> [1] "4" "5" "7"
#> attr(,"gate")
#>         k2       k1
#> 1 6.099191 8.274120
#> 2 8.129107 7.048649
#> 3 8.526881 5.859404
#> 4 5.700760 6.716428
#> 5 5.605314 5.953430
#> 6 6.866882 3.764390
#> 7 3.313575 3.344069
#> 8 2.417270 5.217868

Answer 2

locator {graphics}  R Documentation
Graphical Input

Description

Reads the position of the graphics cursor when the (first) mouse button is pressed.

![> pts <- locator(4)
> polygon(pts)
> png(); plot(1,1)
> pts <- locator(4)
> polygon(pts)
> dev.off()][1]

Answer 3

Try something like this, since identify returns the seq_along(x) for the point that you click near (what you refer to as 'some meaningless number'):

x <- rnorm(10)
y <- rnorm(10)
plot(x,y)
out <- sapply(list(x,y),"[",identify(x,y))
# do some clicking
out
# something like this is returned for the x/y points
#            [,1]        [,2]
#[1,] -0.62221766 -0.73838314
#[2,] -0.69896643  0.40186536
#[3,]  0.06077831 -1.63940474
#[4,] -0.09900270  0.00062011

Answer 4

The key is using the result as an index. This can then be used to identify the specific xy coordinates:

n <- 10
x <- runif(n)
y <- runif(n)
df <- data.frame(x=x, y=y)

plot(y ~ x, data=df)
df[identify(x, y, n=1),]