How to interpolate two colors in a cell?

Question

I just want to fill a cell with a two colors range like a heatmap, see the figure bellow.

Answer 1

This question appealed to me: how do you automatically colour the entries of a table based on the values in the cells? That is, how do we make the not-so-random table

\begin{tabular}{ *7H }
-1.0 & -0.9 & -0.8 & -0.7 & -0.6 & -0.5 & -0.4 \\
-0.3 & -0.2 & -0.1 &  0.0 &  0.1 &  0.2 &  0.3 \\
 0.4 &  0.5 &  0.6 &  0.7 &  0.8 &  0.9 &  1.0 \\
\multicolumn{7}{c}{Min
   \tikz{\shade[left color=blue,right color=red] 
          (current page.south west) rectangle ++(#1,12pt);}
    Max}
\end{tabular}

produce the following?

[The OP is perhaps mainly asking about drawing a gradient shading, but as John Kormylo says in the comments (and shown above), this is easy to do using tikz.]

In answering this I assumed that these matrices have entries between -1 and 1 -- it is easy to change the code below to work for other domains. To automatically assign a background colour to each cell based on the value of the entry I first use the array package to define a new column type

\newcolumntype{H}{>{\collectcell\Heat}r<{\endcollectcell}}

In turn, the H column type uses the collcell package to apply the the command \Heat to each cell entry. The \Heat macro first maps the cell entry to an integer between 0 and 100 using pgfmath

\pgfmathparse{int(50+50*#1)}% map number in [-1,1] to [0,100]

This number is then used to set the background colour of the cell with \cellcolor{red!\pgfmathresult!blue} -- although, in practice a little expansion hackery is needed here. With these definitions in place the code above does what I wanted.

Here is the full code:

\documentclass{article}
\usepackage[table]{xcolor}
\usepackage{collcell}
\usepackage{array}
\usepackage{tikz}
\newcolumntype{H}{>{\collectcell\Heat}r<{\endcollectcell}}

\newcommand\Heat[1]{% \Heat{number in the interval [-1,1]}
    \pgfmathparse{int(50+50*#1)}% map number in [-1,1] to [0,100]
    \edef\HeatCell{\noexpand\cellcolor{red!\pgfmathresult!blue}}%
    \HeatCell$#1$%
}

\begin{document}

\begin{tabular}{ *7H }
-1.0 & -0.9 & -0.8 & -0.7 & -0.6 & -0.5 & -0.4 \\
-0.3 & -0.2 & -0.1 &  0.0 &  0.1 &  0.2 &  0.3 \\
 0.4 &  0.5 &  0.6 &  0.7 &  0.8 &  0.9 &  1.0 \\
\multicolumn{7}{c}{Min
   \tikz{\shade [left color=blue ,right color=red]
         (current page.south west) rectangle ++(#1,12pt);}
    Max}
\end{tabular}

\end{document}

This said, the choice of colours makes the table very garish and unpleasant to look at. A first improvement would be to make the text colour white, which we can do by changing \Heat to:

\newcommand\Heat[1]{% \Heat{number in the interval [-1,1]}
    \pgfmathparse{int(50+50*#1)}% map number in [-1,1] to [0,100]
    \edef\HeatCell{\noexpand\cellcolor{red!\pgfmathresult!blue}}%
    \HeatCell\textcolor{white}{$#1$}%
}

This produces:

We could instead change the colours to Yellow1 and SpringGreen3 (using the x11names option to xcolor), to get:

Edit

Just for fun here's the same code but with a pgfkeys interface for setting the colours and the min/max values via \Heatset{min colour=blue, max colour=red} etc. There is also a slider \Heatset{slider= <width> } for the gradient bar:

\documentclass{article}
\usepackage[table,x11names]{xcolor}
\usepackage{collcell}
\usepackage{array}
\usepackage{tikz}
\usepackage{pgfkeys}

% set up pgfkeys for controlling heat specifications
\pgfkeys{/heat/.is family, /heat,
    min colour/.initial = Yellow1,
    max colour/.initial = SpringGreen3,
    text colour/.initial = black,
    min color/.style = {min colour=#1},% for our friends who can't spell
    max color/.style = {max colour=#1},
    text color/.style = {text colour=#1},
    min/.initial = -1,
    max/.initial = 1,
    slider/.code={%
       \tikz{\shade[left color=\HVal{min colour},%
                    right color=\HVal{max colour}]%
          (current page.south west) rectangle ++(#1,12pt);
       }%
    }%
}
\newcommand\Heatset[1]{\pgfkeys{/heat, #1}}
\newcommand\HVal[1]{\pgfkeysvalueof{/heat/#1}}

\newcolumntype{H}{>{\collectcell\Heat}r<{\endcollectcell}}
\newcommand\Heat[1]{% \Heat{number in the interval [min, max] }
  \pgfmathparse{int(100*(#1-\HVal{min})/(\HVal{max}-\HVal{min}))}% map number to [0,100]
  \edef\HeatCell{\noexpand\cellcolor{\HVal{max colour}!\pgfmathresult!\HVal{min colour}}}%
  \HeatCell\textcolor{\HVal{text colour}}{$#1$}%
}

\begin{document}

\begin{tabular}{ *7H }
-1.0 & -0.9 & -0.8 & -0.7 & -0.6 & -0.5 & -0.4 \\
-0.3 & -0.2 & -0.1 &  0.0 &  0.1 &  0.2 &  0.3 \\
 0.4 &  0.5 &  0.6 &  0.7 &  0.8 &  0.9 &  1.0 \\
 \multicolumn{7}{c}{Min \Heatset{slider=5.5} Max}
\end{tabular}

\bigskip\noindent
\Heatset{max colour=red, min colour=blue, min=0, max=100, text colour=white}
\verb|\Heatset{max colour=red, min colour=blue, min=0, max=100, text colour=white}|
\begin{tabular}{ *7H }
 10 &  90 &  80 &  70 &  60 &  50 &  40 \\
 30 &  20 &  10 &  00 &  10 &  20 &  30 \\
 40 &  50 &  60 &  70 &  80 &  90 &  10 \\
 \multicolumn{7}{c}{Min \Heatset{slider=3.0} Max}
\end{tabular}

\end{document}

This produces: