[R] Colours for 3-way probabilities

[Earl F Glynn]

Carl Witthoft wrote:
> There are a couple Venn Diagram functions out there.
>
> But I would strongly recommend against making charts like this. There
> are too many colors, and even non-colorblind people will find them to be
> a pain to discriminate, let alone remember what the coding means.
>
> Assuming you want to show the distribution on a cartographic map, maybe
> a mini-barchart in each state or county would be better, or three
> non-overlapping 'bubbles' whose diameter or area maps to votecount.
>
> Tufte has written a bunch about this sort of problem.
>
>
> <quote>
> Are there any R functions for creating palettes for three-way data? For
> example, election maps for three parties where pure red, blue, and green
> show 100% for the Red, Blue, and Green parties respectively, magenta
> shows a 50-50 Red-Blue split with 0 for the Greens, cyan a 50-50
> Blue/Green split with no Red votes and so on, with grey, black or white
> at a 1/3,1/3,1/3 split vote.

I'm not sure that I fully understand what you're trying to do.

A chromaticity diagram conceptually shows all possible colors.
http://www.efg2.com/Lab/Graphics/Colors/Chromaticity.htm

With older CRT displays, the chromaticity coordinates of the red, green 
and blue phosphors on a chromaticity chart was a triangular "color 
gamut" that device could display.

The gamuts of newer devices, such as LCD displays, or multi-ink printers 
are a bit harder to explain, but basically could be thought of as some 
sort of polygon of possible colors on a chromaticity chart.  Color 
coordinates outside a gamut cannot be displayed on a device.

In theory, devices can have different color gamuts.  There's no 
guarantee that a color outside of the gamut of a display device will be 
displayed as seen on the original device.

The most common additive color primaries are red-green-blue and for 
displays, it's common to use the rgb function to define colors.

See RGB color space here:
http://en.wikipedia.org/wiki/RGB_color_space

R's rgb function (?rgb) by default allows the definition of colors on a 
continuum of 0.0 to 1.0 of R, G, and B primaries.  E.g., red is rgb(1.0, 
0.0, 0.0) and blue would be rgb(0.0, 0.0, 1.0).

So, if you're working with 0.0 to 1.0 probabilities they could be mapped 
the RGB coordinates.

But the reality is that most devices are 24-bit color (ignoring the 
8-bit "alpha" channel).  R's rgb function can be called as rgb(255,0,0, 
maxColorValue=255) to define "red", or rgb(0,0,255, maxColorValue=255) 
to define "blue".  Such 0:255 ranges map directly to the hardware in 
many cases.

This chart shows the result of fully-saturated additive RGB primary 
colors:  http://www.efg2.com/Lab/Graphics/Colors/ColorMix.htm
[Also note the subtractive color primaries mentioned there.]

"Shades of gray" are produced when the R,G,B components are all the 
same, e.g., rgb(0,0,0) is black and rgb(1.0, 1.0, 1.0) is white.

In theory, you could define any three additive color primaries and use a 
Maxwell Triangle of the possible colors produced, e.g., with RGB:
http://www.efg2.com/Lab/Graphics/Colors/MaxwellTriangle.htm

The above discussion works well for continuous data combinations, but if 
you're working with categorical data, perhaps just pick a categorical 
color scheme.  See: Color Schemes Appropriate for Scientific Data Graphics,
http://geography.uoregon.edu/datagraphics/color_scales.htm

This page may be useful for picking R colors:
http://research.stowers-institute.org/efg/R/Color/Chart/index.htm

Or use this PDF showing R's named colors:
http://research.stowers-institute.org/efg/R/Color/Chart/ColorChart.pdf

Besides RGB, other color spaces may be helpful for color selection and 
display.  Hue-Saturtion-Value coordinates can be useful with some 
problems:  See http://en.wikipedia.org/wiki/HSL_and_HSV and 
http://www.efg2.com/Lab/Graphics/Colors/HSV.htm.

For info on R's hsv function:  ?hsv

This page shows the "hue" map of an RGB image:
http://www.efg2.com/Lab/Graphics/Colors/ShowImage.htm

See the Color Chart page above for a brief example of using R's rgb2hsv 
function to convert from RGB to HSV.

efg

Earl F Glynn
Overland Park, KS