Panel functions for level-coded irregular points

These panel functions for levelplot can represent irregular (x, y) points with a color covariate. panel.levelplot.points simply draws color-coded points. panel.voronoi uses the interp or deldir package to calculate the spatial extension of a set of points in 2 dimensions. This is known variously as a Voronoi mosaic, a Dirichlet tesselation, or Thiessen polygons.

panel.voronoi(x, y, z, subscripts = TRUE, at = pretty(z),
   points = TRUE, border = "transparent",
   na.rm = FALSE, win.expand = 0.07, use.tripack = FALSE,
   backend = c("interp", "deldir"),
   ...,
   col.regions = regions$col, alpha.regions = regions$alpha)

panel.levelplot.points(x, y, z, subscripts = TRUE, at = pretty(z),
   shrink, labels, label.style, contour, region,
   pch = 21, col.symbol = "#00000044",
   ...,
   col.regions = regions$col, fill = NULL)

Arguments

x, y, z

an irregular set of points at locations (x, y) with value z.

subscripts

integer vector indicating what subset of x, y and z to draw. Typically passed by levelplot.

at, col.regions, alpha.regions

color scale definition; see panel.levelplot.

points

whether to draw the (x, y) points.

border

color for polygon borders.

na.rm

if TRUE, points with missing z values will be excluded from the calculation of polygons. If FALSE, those polygons are calculated but are not drawn (i.e. are transparent).

win.expand

Defines the rectangular window bounding the polygons. This is a factor by which to expand the range of the data. Set to 0 to limit drawing at the furthest data point locations. Applies only for the "deldir" backend (see Details).

use.tripack

Ignored, with a warning if a value is specified. See Details.

backend

Character string specifying backend package to use to calculate the Voronoi mosaic. Defaults to "interp".

...

Additional arguments; passed to panel.xyplot if points = TRUE.

pch, col.symbol

symbol and border color for points. A filled symbol should be used, i.e. in the range 21-25.

shrink

Ignored.

labels

Ignored.

label.style

Ignored.

contour

Ignored.

region

Ignored.

fill

Ignored.

Details

Up to version 0.6-29, latticeExtra used the deldir package to compute the Voronoi mosaic by default, while offering the choice of using the faster but non-free ACM licensed implementation in the tripack package by setting use.tripack = TRUE. Later versions use the FOSS replacements in interp by default, and deprecates the use.tripack argument. The deldir implementation, which allows polygons to be clipped to a rectangular window (the win.expand argument), can still be used using backend = "deldir".

Author

Felix Andrews felix@nfrac.org

See also

tileplot, panel.levelplot, deldir

Examples

## a variant of Figure 5.6 from Sarkar (2008)
## http://lmdvr.r-forge.r-project.org/figures/figures.html?chapter=05;figure=05_06

depth.ord <- rev(order(quakes$depth))
quakes$Magnitude <- equal.count(quakes$mag, 4)
quakes.ordered <- quakes[depth.ord, ]

levelplot(depth ~ long + lat | Magnitude, data = quakes.ordered,
         panel = panel.levelplot.points, type = c("p", "g"),
         aspect = "iso", prepanel = prepanel.default.xyplot)


## a levelplot with jittered cells

xyz <- expand.grid(x = 0:9, y = 0:9)
xyz[] <- jitter(as.matrix(xyz))
xyz$z <- with(xyz, sqrt((x - 5)^2 + (y - 5)^2))
levelplot(z ~ x * y, xyz, panel = panel.voronoi, points = FALSE)


## hexagonal cells

xyz$y <- xyz$y + c(0, 0.5)
levelplot(z ~ x * y, xyz, panel = panel.voronoi, points = FALSE)