label.ellipse: Label an ellipse

Description

label.ellipse is used to a draw text label on an ellipse at its center or somewhere around the periphery in a very flexible way.

Usage

label.ellipse(
  ellipse,
  label,
  col = "black",
  label.pos = NULL,
  xpd = TRUE,
  tweak = 0.5 * c(strwidth("M"), strheight("M")),
  ...
)

Arguments

ellipse: A two-column matrix of coordinates for the ellipse boundary
label: Character string to be used as the ellipse label
col: Label color
label.pos: Label position relative to the ellipse. See details
xpd: Should the label be allowed to extend beyond the plot limits?
tweak: A vector of two lengths used to tweak label positions
...: Other parameters passed to text, e.g., cex, ...

Author

Michael Friendly

Details

If label.pos=NULL, the function uses the sign of the correlation represented by the ellipse to determine a position at the top (\(r>=0\)) or bottom (\(r<0\)) of the ellipse. Integer values of 0, 1, 2, 3 and 4, respectively indicate positions at the center, below, to the left of, above and to the right of the max/min coordinates of the ellipse. Label positions can also be specified as the corresponding character strings c("center", "bottom", "left", "top", "right"), or compass directions, c("C", "S", "W", "N", "E"), or Other integer label.pos values, 5:nrow(ellipse) are taken as indices of the row coordinates to be used for the ellipse label. Equivalently, label.pos can also be a fraction in (0,1), interpreted as the fraction of the way around the unit circle, counterclockwise from the point (1,0).

Examples

Run this code

circle <- function(center=c(0,0), radius=1, segments=60) {
   angles <- (0:segments)*2*pi/segments
   circle <- radius * cbind( cos(angles), sin(angles))
   t( c(center) + t( circle ))
}

label_demo <- function(ell) {
  plot(-2:2, -2:2, type="n", asp=1, main="label.pos values and points (0:60)")
  lines(ell, col="gray")
  points(0, 0, pch="+", cex=2)
  
  labs <- c("center", "bot", "left", "top", "right")
  for (i in 0:4) {
    label.ellipse(ell, label=paste(i, ":", labs[i+1]), label.pos = i)
  }
  for( i in 5*c(1,2, 4,5, 7,8, 10,11)) {
    points(ell[i,1], ell[i,2], pch=16)
    label.ellipse(ell, label=i, label.pos=i)
  }
}

circ <- circle(radius=1.8)
label_demo(circ)

ell <-circ %*% chol(matrix( c(1, .5, .5, 1), 2, 2)) 
label_demo(ell)