R: Identify Points in a Scatter Plot

identify {graphics} | R Documentation

## Identify Points in a Scatter Plot

### Description

`identify` reads the position of the graphics pointer when the (first) mouse button is pressed. It then searches the coordinates given in `x` and `y` for the point closest to the pointer. If this point is close enough to the pointer, its index will be returned as part of the value of the call.

### Usage

    identify(x, ...)

    ## Default S3 method:
    identify(x, y = NULL, labels = seq_along(x), pos = FALSE,
             n = length(x), plot = TRUE, atpen = FALSE, offset = 0.5,
             tolerance = 0.25, ...)

### Arguments

`x, y` | coordinates of points in a scatter plot. Alternatively, any object which defines coordinates (a plotting structure, time series etc: see `xy.coords`) can be given as `x`, and `y` left missing.
`labels` | an optional character vector giving labels for the points. Will be coerced using `as.character`, and recycled if necessary to the length of `x`. Excess labels will be discarded, with a warning.
`pos` | if `pos` is `TRUE`, a component is added to the return value which indicates where text was plotted relative to each identified point: see Value.
`n` | the maximum number of points to be identified.
`plot` | logical: if `plot` is `TRUE`, the labels are printed near the points and if `FALSE` they are omitted.
`atpen` | logical: if `TRUE` and `plot = TRUE`, the lower-left corners of the labels are plotted at the points clicked rather than relative to the points.
`offset` | the distance (in character widths) which separates the label from identified points. Negative values are allowed. Not used if `atpen = TRUE`.
`tolerance` | the maximal distance (in inches) for the pointer to be ‘close enough’ to a point.
`...` | further arguments passed to `par` such as `cex`, `col` and `font`.

### Details

`identify` is a generic function, and only the default method is described here.

`identify` is only supported on screen devices such as `X11`, `windows` and `quartz`. On other devices the call will do nothing.

Clicking near (as defined by `tolerance`) a point adds it to the list of identified points. Points can be identified only once, and if the point has already been identified or the click is not near any of the points a message is printed immediately on the R console.

If `plot` is `TRUE`, the point is labelled with the corresponding element of `labels`. If `atpen` is false (the default) the labels are placed below, to the left, above or to the right of the identified point, depending on where the pointer was relative to the point. If `atpen` is true, the labels are placed with the bottom left of the string's box at the pointer.

The identification process is terminated by clicking the second button and selecting ‘Stop’ from the menu, or from the ‘Stop’ menu on the graphics window.

On most devices which support `identify`, successful selection of a point is indicated by a bell sound unless `options(locatorBell = FALSE)` has been set.

If the window is resized or hidden and then exposed before the identification process has terminated, any labels drawn by `identify` will disappear. These will reappear once the identification process has terminated and the window is resized or hidden and exposed again. This is because the labels drawn by `identify` are not recorded in the device's display list until the identification process has terminated.

If you interrupt the `identify` call this leaves the graphics device in an undefined state, with points labelled but labels not recorded in the display list. Copying a device in that state (e.g., saving the plot from the window's menu) will give unpredictable results.

### Value

If `pos` is `FALSE`, an integer vector containing the indices of the identified points, in the order they were identified.

If `pos` is `TRUE`, a list containing a component `ind`, indicating which points were identified and a component `pos`, indicating where the labels were placed relative to the identified points (1=below, 2=left, 3=above, 4=right and 0=no offset, used if `atpen = TRUE`).

### Technicalities

The algorithm used for placing labels is the same as used by `text` if `pos` is specified there, the difference being that the position of the pointer relative the identified point determines `pos` in `identify`.

For labels placed to the left of a point, the right-hand edge of the string's box is placed `offset` units to the left of the point, and analogously for points to the right. The baseline of the text is placed below the point so as to approximately centre string vertically. For labels placed above or below a point, the string is centered horizontally on the point. For labels placed above, the baseline of the text is placed `offset` units above the point, and for those placed below, the baseline is placed so that the top of the string's box is approximately `offset` units below the point. If you want more precise placement (e.g., centering) use `plot = FALSE` and plot via `text` or `points`: see the examples.

### References

Becker, R. A., Chambers, J. M. and Wilks, A. R. (1988) The New S Language. Wadsworth & Brooks/Cole.

### See Also

`locator`, `text`.

`dev.capabilities` to see if it is supported.

### Examples

    ## A function to use identify to select points, and overplot the
    ## points with another symbol as they are selected
    identifyPch <- function(x, y = NULL, n = length(x), plot = FALSE, pch = 19, ...)
    {
        xy <- xy.coords(x, y); x <- xy$x; y <- xy$y
        sel <- rep(FALSE, length(x))
        while(sum(sel) < n) {
            ans <- identify(x[!sel], y[!sel], labels = which(!sel), n = 1, plot = plot, ...)
            if(!length(ans)) break
            ans <- which(!sel)[ans]
            points(x[ans], y[ans], pch = pch)
            sel[ans] <- TRUE
        }
        ## return indices of selected points
        which(sel)
    }

    if(dev.interactive()) { ## use it
      x <- rnorm(50); y <- rnorm(50)
      plot(x,y); identifyPch(x,y) # how fast to get all?
    }

[Package graphics version 3.5.1 Index]