syms: Function to Compute the Diameters of Proportional Symbols

Description

This function computes the diameters of the open circles to be plotted in a map or other display.

Usage

syms(z, zrange = c(NA, NA), p = 1)

Arguments

name of the variable to be plotted for which diameters are to be computed.

zrange

The mimimum and maximum values of z to be used as the lower and upper limits, respectively, for the computed symbol diameters. By default the minimum and maximum values of the input data are used.

a parameter that controls the rate of change of symbol diameter with changing value. A default of p = 1 is provided that results in a linear rate of change. See Details below.

Value

zdiam

the computed diameter of the symbol.

Details

The symbol diameter is computed as a function of the value z to be plotted: diameter = dmin + (dmax - dmin) * {(z - zmin)/(zmax - zmin)}^p where dmin and dmax are defined as 0.1 and 1 units, so the symbol diameters range over an order of magnitude (and symbol areas over two); zmin and zmax are the observed range of the data, or the range over which the user wants the diameters to be computed; and p is a power defined by the user. The value of (z - zmin)/(zmax - zmin) is the value of z normalized, 0 - 1, to the range over which the symbol diameters are to be computed. After being raised to the power p, which will result in a number in the range 0 to 1, this value is multiplied by the permissable range of diameters and added to the minimum diameter. This results in a diameter between 0.1 and 1 units that is proportional to the value of z.

A p value of 1 results in a linear rate of change. Values of p less than unity lead to a rapid intial rate of change with increasing value of z which is often suitable for displaying negatively skewed data sets, see the example below. In contrast, values of p greater than unity result in an initial slow rate of change with increasing value of z which is often suitable for displaying positively skewed data sets. Experimentation is usually necessary to obtain a satisfactory visual effect. See syms.pfunc for a graphic demonstrating the effect of varying the p parameter.

If zmin or zmax are defined this has the effect of setting a minimum or maximum value of z, respectively, beyond which changes in the value of z do not result in changes in symbol diameter. This can be useful in limiting the effect of one or a few extreme outliers while still plotting them, they simply plot at the minimum or maximum symbol size and are not involved in the calculation of the range of z over which the diameter varies.

Examples

Run this code

# NOT RUN {
## Make test data available
data(kola.o)
attach(kola.o)

## Compute default symbol diameters
circle.diam <- syms(Cu, p = 0.3)
circle.diam

## Compute symbol diameters holding all symbols for values greater 
## than 1000 to the same size
circle.diam <- syms(Cu, zrange = c(NA, 1000), p = 0.3)
circle.diam 

## Clean-up and detach test data
rm(circle.diam)
detach(kola.o)
# }