Generating TikZ Nodes and Labels from Custom Array Strings

Question

I am trying to use the TikZ environment to annotate a graphic with labels in multiple specific locations. Instead of repeating the node command many times and changing the node location and label, I would really like for TikZ to use coordinates and labels from custom strings that I input, along with the \foreach command.

Also, these coordinates should be given as dimensionless decimal scalars (0-1) of the total graphic width and height, so that the nodes are in respective locations even if total width/height changes. I get the scaled figure's height from specified width thanks to Will Robertson's answer to a question here.

I have a non-functioning MWE here which should take this first image and add labels like in the second. Perhaps someone could tell me what I'm doing wrong? Thanks in advance!

\documentclass{article}
\usepackage[utf8]{inputenc}
\usepackage{tikz}
\usepackage{pgfmath,pgffor}
\usepackage{calc}

\def\site{{1,2,3,4,5}} %Load labels
\def\xdim{{0.038,0.149,0.488,0.668,0.872
}} %Load dimensionless x-coordinates for labels
\def\ydim{{0.57,0.908,0.713,0.632,0.688
}} %Load dimensionless y-coordinates for labels

\begin{document}
    \newlength\gw
    \setlength\gw{10cm} %Load graphic width
    \def\mygraphic{\includegraphics[width=\gw]{label_this.jpg}}
    \newlength\gh
    \setlength\gh{\heightof{\mygraphic}} %Graphic height
    \begin{figure}
        \centering
        \begin{tikzpicture}
            \node at (0,0) {\includegraphics[width=\gw]{label_this.jpg}};
            \foreach \i in {0,...,4}
            {
            \node at (\xdim[\i]\gw,\ydim[\i]\gh) {\site[\i]};
            }
        \end{tikzpicture}
        \caption{Labelled Graphic}
    \end{figure}
\end{document}

Answer 1

You were almost there. You forgot multiplication signs, forgot to parse the node content, and do not need the calc package, the library of the same name allows you to do the same.

\documentclass{article}
\usepackage[utf8]{inputenc}
\usepackage{tikz}
\usetikzlibrary{calc}

\def\site{{1,2,3,4,5}} %Load labels
\def\xdim{{0.038,0.149,0.488,0.668,0.872
}} %Load dimensionless x-coordinates for labels
\def\ydim{{0.57,0.908,0.713,0.632,0.688
}} %Load dimensionless y-coordinates for labels

\begin{document}
    \newlength\gw
    \setlength\gw{10cm} %Load graphic width
    \def\mygraphic{\includegraphics[width=\gw]{label_this.jpg}}
%     \newlength\gh
%     \setlength\gh{\heightof{\mygraphic}} %Graphic height
    \begin{figure}
        \centering
        \begin{tikzpicture}
            \node[inner sep=0pt] (grph) at (0,0) {\includegraphics[width=\gw]{label_this.jpg}};
            \path let \p1=($(grph.north)-(grph.south)$) in
            \pgfextra{\xdef\gh{\y1}};
            \foreach \i in {0,...,4}
            {
            \node at (\xdim[\i]*\gw-4.5cm,\ydim[\i]*\gh-2.5cm) {\pgfmathparse{\site[\i]}%
            \pgfmathresult};
            }
        \end{tikzpicture}
        \caption{Labelled Graphic}
    \end{figure}
\end{document}