R: Label placement for points to avoid overlaps

pointLabel {maptools} | R Documentation

## Label placement for points to avoid overlaps

### Description

Use optimization routines to find good locations for point labels without overlaps.

### Usage

    pointLabel(x, y = NULL, labels = seq(along = x), cex = 1,
               method = c("SANN", "GA"),
               allowSmallOverlap = FALSE,
               trace = FALSE,
               doPlot = TRUE,
               ...)

### Arguments

`x, y` | as with `plot.default`, these provide the x and y coordinates for the point labels. Any reasonable way of defining the coordinates is acceptable. See the function `xy.coords` for details.
`labels` | as with `text`, a character vector or expression specifying the text to be written. An attempt is made to coerce other language objects (names and calls) to expressions, and vectors and other classed objects to character vectors by `as.character`.
`cex` | numeric character expansion factor as with `text`.
`method` | the optimization method, either “SANN” for simulated annealing (the default) or “GA” for a genetic algorithm.
`allowSmallOverlap` | logical; if `TRUE`, labels are allowed a small overlap. The overlap allowed is 2% of the diagonal distance of the plot area.
`trace` | logical; if `TRUE`, status updates are given as the optimization algorithms progress.
`doPlot` | logical; if `TRUE`, the labels are plotted on the existing graph with `text`.
`...` | arguments passed along to `text` to specify labeling parameters such as `col`.

### Details

Eight positions are candidates for label placement, either horizontally, vertically, or diagonally offset from the points. The default position for labels is the top right diagonal relative to the point (considered the preferred label position).

With the default settings, simulating annealing solves faster than the genetic algorithm. It is an open question as to which settles into a global optimum the best (both algorithms have parameters that may be tweaked).

The label positioning problem is NP-hard (nondeterministic polynomial-time hard). Placement becomes difficult and slows considerably with large numbers of points. This function places all labels, whether overlaps occur or not. Some placement algorithms remove labels that overlap.

Note that only `cex` is used to calculate string width and height (using `strwidth` and `strheight`), so passing a different font may corrupt the label dimensions. You could get around this by adjusting the font parameters with `par` prior to running this function.

### Value

An `xy` list giving the `x` and `y` positions of the label as would be placed by `text(xy, labels)`.

### Author(s)

Tom Short, EPRI, tshort@epri.com

### References

http://en.wikipedia.org/wiki/Automatic_label_placement

http://i11www.iti.uni-karlsruhe.de/map-labeling/bibliography/

http://www.eecs.harvard.edu/~shieber/Projects/Carto/carto.html

http://www.szoraster.com/Cartography/PracticalExperience.htm

The genetic algorithm code was adapted from the python code at

http://meta.wikimedia.org/wiki/Map_generator.

The simulated annealing code follows the algorithm and guidelines in:

Jon Christensen, Joe Marks, and Stuart Shieber. Placing text labels on maps and diagrams. In Paul Heckbert, editor, Graphics Gems IV, pages 497-504. Academic Press, Boston, MA, 1994. http://www.eecs.harvard.edu/~shieber/Biblio/Papers/jc.label.pdf

### See Also

`text`, `thigmophobe.labels` in package plotrix

### Examples

    n <- 50
    x <- rnorm(n)*10
    y <- rnorm(n)*10
    plot(x, y, col = "red", pch = 20)
    pointLabel(x, y, as.character(round(x,5)), offset = 0, cex = .7)

    plot(x, y, col = "red", pch = 20)
    pointLabel(x, y, expression(over(alpha, beta[123])), offset = 0, cex = .8)

[Package maptools version 0.9-3 Index]