How to keep hue from going beyond 300° in xcolor/pgf? (Doing math in pgfplotstable)

Question

Ok, this is probably an incredibly trivial question for anyone who knows more about xcolor or pgf or TikZ or whatever really. I tried to use a hue scale for a heat map/color matrix with values 0<=x<=1. I based this heavily on this answer by adn:

\documentclass{article}
\usepackage[table]{xcolor}
\usepackage{pgfplotstable}

\usepackage{xstring}

\pgfplotstableset{
    color cells/.style={
        col sep=comma,
        string type,
        postproc cell content/.code={%
                \pgfkeysalso{@cell content=\rule{0cm}{2.4ex}\IfDecimal{##1}{%
                \cellcolor[hsb]{##1,.7,1.}}{}##1}%
                },
        columns/x/.style={
            column name={},
            postproc cell content/.code={}
        }
    }
}

\begin{document}
\begin{table}\caption{Correlation or something}
\centering
\pgfplotstabletypeset[color cells]{
x,a,b,c,d
a,0.9,0.3,-.1,0
b,0,.8,.6,.4
c,1,0.2,.95,.5
d,0,.1,.7,.85
}
\end{table}
\end{document}

This looks like this:

Colorful, but it does what it's supposed to do, even for small negative numbers which aren't supposed to exist ... except:

For a value of 1 the hue wraps around and is back to 0/red again. I tried to subtract a tiny value or multiply the cell content internally by .99 or something but I simply failed to do any arithmetic here.

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PS: If you can recommend any good documentation for xcolor, pgf and the like, that'd be great, too. Maybe next time I won't have to ask such stupid questions then.

---

Edit:

Ok, I didn't do my homework very well. The HSB color space doesn't suddenly wrap around. It is actually circular. This means that purple (RGB [1, 0, 1]) is at 300° which according to my calculations is the very handy number .8333... in decimal. So one would have to map one interval to another which in this case should be rather simple: just multiply the value by .8333.

Answer 1

I'm pretty sure I don't understand the question but this forces things that are >=1 to 0.99 and things that are <= 0 to 0.01.

\pgfplotstableset{
    color cells/.style={
        col sep=comma,
        string type,
        postproc cell content/.code={%
                \pgfkeysalso{@cell content=\rule{0cm}{2.4ex}\IfDecimal{##1}{%
      \edef\tmp{\noexpand\cellcolor[hsb]{%
       \ifdim##1pt>0pt  \ifdim##1pt<1pt ##1 \else 0.99 \fi \else 0.01\fi,.7,1.}}%
                \tmp}{}##1}%
                },
        columns/x/.style={
            column name={},
            postproc cell content/.code={}
        }
    }
}

Answer 2

For the arithmetics, you can check if the argument is negative to see whether a max or min should be performed and then you can use the \pgfmathresult in the \cellcolor. But it's value should be expanded before \cellcolor see it as a macro. Hence along the lines of David's answer you can hide it into a expanding definition, expand it and then execute it.

\documentclass{article}
\usepackage[table]{xcolor}
\usepackage{pgfplotstable}

\usepackage{xstring}

\pgfplotstableset{
    color cells/.style={
        col sep=comma,
        string type,
        postproc cell content/.code={%
                \pgfkeysalso{@cell content=\rule{0cm}{2.4ex}%
                         \pgfmathparse{##1<0?max(0,##1):min(0.99,##1)}%
                         \edef\x{\noexpand\cellcolor[hsb]{\pgfmathresult,0.7,1.0}}\x##1%
            }
        },
        columns/x/.style={
            column name={},
            postproc cell content/.code={}
        }
    }
}

\begin{document}
\begin{table}\caption{Correlation or something}
\centering
\pgfplotstabletypeset[color cells]{
x,a,b,c,d
a,0.9,0.3,-.1,0
b,0,.8,.6,.4
c,1,0.2,.95,.5
d,0,.1,.7,.85
}
\end{table}
\end{document}

Then the result is hopefully correct (Colorblind here). There is nothing wrong with the question by the way. Here are the manuals for all packages :-) TeXdoc.net

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Edit

The code can easily be adapted to fit the changed question like this:

\pgfplotstableset{
    color cells/.style={
        col sep=comma,
        string type,
        postproc cell content/.code={%
                \pgfkeysalso{@cell content=\rule{0cm}{2.4ex}%
                         \pgfmathparse{0.8333*##1}%
                         \edef\x{\noexpand\cellcolor[hsb]{\pgfmathresult,0.7,1.0}}\x##1%
            }
        },
        columns/x/.style={
            column name={},
            postproc cell content/.code={}
        }
    }
}

The result then looks like this: