R: Display the Density of Points in a Scatter Plot by Colors

Image {IDPmisc} | R Documentation

## 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

    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

    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)

[Package IDPmisc version 1.1.20 Index]