pgfplots: how to load distinct symbolic ticks/labels from file?

Question

I want to plot a matrix of color-values from a csv file. Inspired by this website, I set out to write my MWE below, to which I added this way of dynamically loading symbolic ticks/labels from my data file.

However, It turns out that my resulting ticks/labels are not distinct, but instead are repeated, like my rendering shows.
Another way to think of it (according to @Jake's comments) is that flexible (x|y)ticklabels from table does not tell PGFplots that these labels are supposed to be symbolic, as doing it manually using symbolic (x|y) coords would.

How can I tell PGFplots that all distinct values from the xitem and yitem columns in my data file are to be used as symbolic labels?
Maybe the problem lies within (x|y) expr=\coordindex. What I want here is to fill in the (symbolic, not numerical) value from the respective colum (xitem|yitem). But using x=xitem,y=yitem, instead gives the error "Package PGF Math Error: Could not parse input 'X' as a floating point number".

Rendering:

MWE:

\documentclass{standalone}

\usepackage{filecontents}
\usepackage{pgfplots, pgfplotstable}

\begin{filecontents}{data.csv}
yitem,xitem,val
r,X,0.1
r,Y,0.2
r,Z,0.3
d,X,0.4
d,Y,0.5
d,Z,0.6
j,X,0.5
j,Y,0.8
j,Z,0.9
\end{filecontents}

\pgfplotstableread[col sep=comma]{data.csv}\datatable
\makeatletter
\pgfplotsset{
    /pgfplots/flexible xticklabels from table/.code n args={3}{
        \pgfplotstableread[#3]{#1}\coordinate@table
        \pgfplotstablegetcolumn{#2}\of{\coordinate@table}\to\pgfplots@xticklabels
        \let\pgfplots@xticklabel=\pgfplots@user@ticklabel@list@x
    },
    /pgfplots/flexible yticklabels from table/.code n args={3}{
        \pgfplotstableread[#3]{#1}\coordinate@table
        \pgfplotstablegetcolumn{#2}\of{\coordinate@table}\to\pgfplots@yticklabels
        \let\pgfplots@yticklabel=\pgfplots@user@ticklabel@list@y
    }
}
\makeatother

\begin{document}
\begin{tikzpicture}
\begin{axis}[
    flexible xticklabels from table={data.csv}{xitem}{col sep=comma},
    xtick=data,
    flexible yticklabels from table={data.csv}{yitem}{col sep=comma},
    ytick=data,
    unbounded coords=jump,
]
\addplot[
    mark=square*,
    only marks,
    scatter,
    point meta=explicit,
] table [
    col sep=comma,
    x expr=\coordindex,
    y expr=\coordindex,
    meta expr=\thisrow{val},
] {\datatable};

\end{axis}
\end{tikzpicture}
\end{document}

Answer 1

You need to generate the mapping from the symbolic coordinates to Cartesian coordinates. If the Cartesian coordinates correspond to the rank of the first appearance of a symbol in a column, you can use the etoolbox package to generate a list that represents the mapping, and then use symbolic x coords to pass that mapping to PGFPlots:

\documentclass[border=5mm]{article}

\usepackage{filecontents}
\usepackage{pgfplots, pgfplotstable}
\usepackage{etoolbox}

\begin{filecontents}{data.csv}
yitem,xitem,val
a,X,0.1
a,Y,0.2
a,Z,0.3
b,X,0.4
b,Y,0.5
b,Z,0.6
c,X,0.5
c,Y,0.8
c,Z,0.9
\end{filecontents}

\pgfplotstableread[col sep=comma]{data.csv}\datatable


\begin{document}
\def\xlistmacro{}
\def\xliststring{}
\pgfplotstableforeachcolumnelement{xitem}\of\datatable\as\entry{%
\xifinlist{\entry}{\xlistmacro}{}{
        \listxadd{\xlistmacro}{\entry}
        \edef\xliststring{\xliststring\entry,}
    }
}

\def\ylistmacro{}
\def\yliststring{}
\pgfplotstableforeachcolumnelement{yitem}\of\datatable\as\entry{%
\xifinlist{\entry}{\ylistmacro}{}{
        \listxadd{\ylistmacro}{\entry}
        \edef\yliststring{\yliststring\entry,}
    }
}

\begin{tikzpicture}
\begin{axis}[
    xtick=data,
    ytick=data,
    symbolic x coords/.expand once={\xliststring},
    symbolic y coords/.expand once={\yliststring},
]
\addplot[
    mark=square*,
    only marks,
    scatter,
    point meta=explicit,
] table [
    col sep=comma,
    meta expr=\thisrow{val},
    x=xitem, y=yitem
] {\datatable};

\end{axis}
\end{tikzpicture}
\end{document}