Add Points to a Plot

Description

points is a generic function to draw a sequence of points at the specified coordinates. The specified character(s) are plotted, centered at the coordinates.

Usage

points(x, ...)

## Default S3 method:
points(x, y = NULL, type = "p", ...)

Arguments

Details

The coordinates can be passed in a plotting structure (a list with x and y components), a two-column matrix, a time series, .... See xy.coords.

Graphical parameters commonly used are

pch

plotting “character”, i.e., symbol to use. This can either be a single character or an integer code for one of a set of graphics symbols. The full set of S symbols is available with pch=0:18, see the last picture from example(points), i.e., the examples below.

In addition, there is a special set of R plotting symbols which can be obtained with pch=19:25 and 21:25 can be colored and filled with different colors:

• pch=19: solid circle,
• pch=20: bullet (smaller circle),
• pch=21: circle,
• pch=22: square,
• pch=23: diamond,
• pch=24: triangle point-up,
• pch=25: triangle point down.

Values pch=26:32 are currently unused, and pch=32:255 give the text symbol in a single-byte locale. In a multi-byte locale such as UTF-8, numeric values of pch greater than or equal to 32 specify a Unicode code point (except for the symbol font as selected by par(font = 5)).

If pch is an integer or character NA or an empty character string, the point is omitted from the plot.

Value pch="." is handled specially. It is a rectangle of side 0.01 inch (scaled by cex). In addition, if cex = 1 (the default), each side is at least one pixel (1/72 inch on the pdf, postscript and xfig devices).

col

color code or name, see par.

bg

background (“fill”) color for the open plot symbols given by pch=21:25.

cex

character (or symbol) expansion: a numerical vector. This works as a multiple of par("cex").

lwd

line width for drawing symbols see par.

Others less commonly used are lty and lwd for types such as "b" and "l".

Graphical parameters pch, col, bg, cex and lwd can be vectors (which will be recycled as needed) giving a value for each point plotted. If lines are to be plotted (e.g. for type = "b" the first element of lwd is used.

Points whose x, y, pch, col or cex value is NA are omitted from the plot.

Note

What is meant by ‘a single character’ is locale-dependent.

The encoding may not have symbols for some or all of the characters in pch=128:255

References

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

See Also

plot, lines, and the underlying workhorse function plot.xy.

Examples

plot(-4:4, -4:4, type = "n")# setting up coord. system
points(rnorm(200), rnorm(200), col = "red")
points(rnorm(100)/2, rnorm(100)/2, col = "blue", cex = 1.5)

op <- par(bg = "light blue")
x <- seq(0,2*pi, len=51)
## something "between type='b' and type='o'":
plot(x, sin(x), type="o", pch=21, bg=par("bg"), col = "blue", cex=.6,
 main='plot(..., type="o", pch=21, bg=par("bg"))')
par(op)

##-------- Showing all the extra & some char graphics symbols ------------
Pex <- 3 ## good for both .Device=="postscript" and "x11"
ipch <- 0:35; np <- length(ipch); k <- floor(sqrt(np)); dd <- c(-1,1)/2
rx <- dd + range(ix <- ipch %/% k)
ry <- dd + range(iy <- 3 + (k-1)- ipch %% k)
pch <- as.list(ipch)
pch[26+ 1:10] <- as.list(c("*",".", "o","O","0","+","-","|","%","#"))
plot(rx, ry, type="n", axes = FALSE, xlab = "", ylab = "",
     main = paste("plot symbols :  points (...  pch = *, cex =", Pex,")"))
abline(v = ix, h = iy, col = "lightgray", lty = "dotted")
for(i in 1:np) {
  pc <- pch[[i]]
  points(ix[i], iy[i], pch = pc, col = "red", bg = "yellow", cex = Pex)
  ## red symbols with a yellow interior (where available)
  text(ix[i] - .3, iy[i], pc, col = "brown", cex = 1.2)
}