Display the Density of Points in a Scatter Plot by Colors

Description

The density of points in a scatter plot is encoded by color.

Usage

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Image(x, y = NULL, pixs = 1, zmax = NULL, ztransf = function(x){x},
      colramp = IDPcolorRamp, factors = c(FALSE, FALSE),
      matrix = FALSE)

Arguments

x, y

Coordinates of points whose density is plotted. If x is a matrix or a data.frame, the first two column are used as x and y respectively. y must be in this case NULL. x and y may be numeric or factor variable.

pixs

Size of pixel in x- and y-direction in [mm] on the plotting device. When x and y are numeric, pixels are square. When x and y are factors or should be handled as factors (see argument factors), pixels are no longer square. The pixels are enlarged in the dimension in which the factors are displayed, so that the rectangular pixels are centered at the factor levels.

zmax

Maximum number of counts per pixel in the plot. When NULL, the density in the scatter plot is encoded from 0 to maximum number of counts per pixel observed. zmax must be equal or larger than maximum number of counts found. The maximum number of counts per pixel is delivered by the return value.

ztransf

Function to transform the number of counts per pixel, which will be mapped by the function in colramp to well defined colors. The user has to make sure that the transformed density lies in the range [0,zmax], where zmax is any positive number (>=2). For examples see ipairs and ilagplot.

colramp

Color ramp to encode the number of the counts within a pixel by color.

factors

Vector of logicals indicating whether x and / or y should be handled as factors independently of their class.

matrix

Boolean. Should all counts be returned in a xyz-matrix or just the maximum.

Details

Before calling Image a plot must have been created by, e.g., calling plot(x,y,type="n"). This function ensures by default that the pixel has the same size in x- and y-direction. As a drawback, pixels may be unequally spaced, when there are only very few distinct (integer) values in x- or y-direction. When this is the case, the corresponding dimension should be declared as a factor. (cf. argument factors).

This function is based on graphics

Value

Maximum number of counts per pixel found (matrix = FALSE) or the full matrix.

Author(s)

Andreas Ruckstuhl, Rene Locher

See Also

ipairs, ilagplot, iplot, image

Examples

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plot.default(iris$Species, iris$Petal.Width, xlim=c(0.5,3.5),
             type="n", axes=FALSE)
axis(1, at=1:3, labels=levels(iris$Species))
axis(2)
Image(iris$Species, iris$Petal.Width, pixs=3)

##
x <- rnorm(10000)
y <- rnorm(10000, 10)
plot(x+y, y, type="n")
Image(x+y, y)
abline(a=0, b=1)

## The above can be merged to
iplot(x+y, y, legend=FALSE, oma=c(5, 4, 4, 2) + 0.1)
abline(a=0, b=1)

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