About replacing period with emdash and aligning caption to figure border

Question

This is about caption package. I read the well-written manual of this package (but not thoroughly, because I am new to LaTex) and found not enough about number(s) associated with caption label. So,

1. How one can substitute the period between chapter and figure numbers with a custom symbol, say an emdash (i.e. as FIGURE 7—1 and without \renewcommand)?
2. Likewise, what about putting a custom symbol right before caption number, something like FIGURE P7—1?

And the third question about caption alignment,

3. Is it possible to align the caption with figure borders (here I mean left border)? How?

---

This is a typical MWE:

\documentclass{book}

\usepackage{blindtext}
\usepackage[demo]{graphicx}

\usepackage{caption}
    \DeclareCaptionStyle{mystyle}[]{
        name=FIGURE,
        labelsep=newline
        }

\setcounter{chapter}{7}

\begin{document}
    \begin{figure}
        \centering
        \includegraphics[width=10cm]{testimage}
        \captionsetup{style=mystyle}
        \caption{Caption of my figure}
    \end{figure}
\end{document}

And the output,

---

Of course I read those questions about using emdash instead of period or caption alignment, but if possible I like to use amsmath and caption package only.

Answer 1

It depends on how you want the references to be printed.

If you want that the reference to figure or table 7–1 is printed as “7.1”, here's a possibility:

\documentclass{book}

\usepackage{graphicx}
\usepackage{caption}

\makeatletter
\DeclareCaptionLabelFormat{pcdn}{% Prefix Chapter Dash Number
  \MakeUppercase{#1} P\arabic{chapter}--\arabic{\@captype}%
}
\makeatother

\DeclareCaptionStyle{mystyle}{
  labelformat=pcdn,
  labelsep=newline
}

\setcounter{chapter}{7}

\begin{document}

We see in figure~\ref{ei}

\begin{figure}[htp]
\captionsetup{style=mystyle}

\centering

\begin{minipage}{10cm}
\includegraphics[width=\textwidth]{example-image}
\caption{Caption of my figure}\label{ei}
\end{minipage}

\end{figure}

\end{document}

The caption package allows defining various formats; in this case we want to change the format for the label (by default “Figure n” or “Table n”, where n is the number). In the second argument to \DeclareCaptionLabelFormat one can use #1 that stands for the appropriate environment name and #2 for the value of the associated counter. Since we want the label in uppercase, \MakeUppercase{#1} is the obvious choice. However since the number should be typeset in a special way, detached from the reference, we can use explicitly the counters, so P\arabic{chapter}--\arabic{figure} might seem the good choice; we can do better, though: in a float environment, \@captype stands either for figure or for table, so \arabic{\@captype} will expand to the needed counter. Since we're using a command with @ in its name, the code must be surrounded by \makeatletter and \makeatother, see What do \makeatletter and \makeatother do?.

---

If the reference should be “P7–1” just like in the caption, the code can be simpler.

\documentclass{book}

\usepackage{graphicx}
\usepackage{caption}

\DeclareCaptionLabelFormat{pcdn}{\MakeUppercase{#1} #2}

\renewcommand{\thefigure}{P\arabic{chapter}--\arabic{figure}}

\DeclareCaptionStyle{mystyle}{
  labelformat=pcdn,
  labelsep=newline
}

\setcounter{chapter}{7}

\begin{document}

We see in figure~\ref{ei}

\begin{figure}[htp]
\captionsetup{style=mystyle}

\centering

\begin{minipage}{10cm}
\includegraphics[width=\textwidth]{example-image}
\caption{Caption of my figure}\label{ei}
\end{minipage}

\end{figure}

\end{document}