Plot Symbols

R allows the user to specify a wide variety of symbols in plots. See ?points for more information.

This script creates a figure containing the standard plot symbols along with some character-based plot symbols. The script is an annotated and slightly modified version of the last example in example(points) from R 2.2.1.

##  Open a default device.

get( getOption( "device" ) )()

##  Pex   expand the symbol size by this amount
##  ipch  vector of plot symbols designated by number
##  np    number of plot symbols drawn in plot
##  k     number of rows of plot symbols
##  dd    c( -0.5, 0.5 )
##  ix    x coordinates for symbols
##  iy    y coordinates for symbols
##  rx    extreme x coordinates for setting up plot
##  ry    extreme y coordinates for setting up plot
##  pch   list of plot symbols

Pex  <- 3           ## good for .Device=="postscript", "x11", "quartz"
ipch <- 0:35
np   <- length( ipch )
k    <- floor( sqrt( np ) )
dd   <- c( -1, 1 ) / 2
ix   <- ipch %/% k
iy   <- 3 + ( k - 1 ) - ipch %% k
rx   <- dd + range( ix )
ry   <- dd + range( iy )
pch  <- as.list( ipch )

##  Values of 0 through 25 specify standard R symbols.
##  Points can also be drawn as text characters; add 10 characters to the
##  list of symbols to be plotted.

pch[ 26 + 1:10 ] <-
    as.list( c( "*", ".", "o", "O", "0", "+", "-", "|", "%", "#" ) )

##  Plot without drawing any points or lines (type = "n").
##  This draws the title and sets up the coordinates of the plot.

plot(
    x    = rx,
    y    = ry,
    type = "n",
    axes = FALSE,
    xlab = "",
    ylab = "",
    main = paste( "plot symbols: points( ..., pch = *, cex =", Pex, ")" ) )

##  Draw horizontal and vertical dotted grid lines.

abline( v = ix, h = iy, col = "lightgray", lty = "dotted" )

##  Iterate through the points, drawing the specified pch (given by an
##  integer from 0 through 25 or by a character ('*' through '#').

for ( i in 1:np ) {
    ##  Extract each plot character from the list using "[[", which
    ##  gets a list component ("[" would return a list).

    pc <- pch[[ i ]]

    ##  Call points() to draw the plot character.
    ##  Draw red symbols with a yellow interior (filled interior is possible
    ##  only for symbols 21 through 25).
    ##  Expand the size of the plot character.

    points(
        x   = ix[ i ],
        y   = iy[ i ],
        pch = pc,
        col = "red",
        bg  = "yellow",
        cex = Pex )

    ##  Label the points with the symbol numbers or characters.

    text(
        x      = ix[ i ] - 0.4,
        y      = iy[ i ],
        labels = pc,
        col    = "brown",
        cex    = 1.2 )
}

The output is:

Plot symbls