[Rd] Colour Schemes

[Barry Rowlingson]

I've been thinking hard about generating colour schemes for data.
There's quite a bit of existing code scattered in various packages for
playing with colours and colour palettes, but I can't find the sort of
thing I'm after for applying colours to data...

To my mind a colour scheme is a mapping from data values to colours.
There's a multitude of such mappings depending on the nature of the
data. For example, for a factor you might want to map levels to unique
colours. For numbers that run from -4 to +2 you might want to use a
diverging colour palette centred on zero. This might be continuous in
some colour space or composed of a small number of discrete colours,
each of which covers a range of values. Or it could be piecewise
continuous as used in topographic maps - less than zero is blue, zero
to 400 goes from sandy yellow to grassy green, 400 to 1000 goes from
grassy green to rocky brown, then suddenly you hit the ice and 1000
and upwards is white.  Or you could have a multivariate mapping where
(x,y,z) -> (r,g,b,a) in complex and non-linear ways.

I see a set of factory functions that return colour scheme mapping
functions that map data to colours, so you'd do:

 # unique colour for each factor level
 scheme1 = exactColours(data$f,someColours)  # data$f is a factor,
someColours is a vector of colour values
 plot(data$x,data$y,col=scheme1(data$f))

 # topological map colouring
 scheme2 = continuousColours(list(-1000,"blue",0,"sandYellow",400,"grassGreen",1000,"rockBrown",1000,"white",10000))
# or something...
 plot(data$x,data$y,col=scheme2(data$height))

Now just because I can't find existing functions like this doesn't
mean they don't exist. There's stuff in plotrix, colorspace,
RColorBrewer etc for creating palettes but then the user is left to
their own devices to map colours to data values.

 Does this kind of thing sound useful? Has it been done? Is it worth
doing? Anybody got any better ideas?

Barry