Drawing an electric dipole with math font Cambria Math

Question

As per the title, I'm trying to draw an electric dipole while utilizing Cambria Math as my math font, in LuaLaTeX. Here's the MWE of my code:

\documentclass{standalone}

\usepackage{unicode-math}
\setmathfont{Cambria Math}

\usepackage{tikz}

\begin{document}
\begin{tikzpicture}
    \draw[gray] (0, 0) -- (4, 0);
    \node[red, draw, circle] at (0, 0) {$+$};
    \node[blue, draw, circle] at (4, 0) {$-$};
\end{tikzpicture}
\end{document}

Which produces the following:

As indicated by the gray line, the + and the - are not on the same line. I've tried many things, including different node anchors, setting text height and text depth, et cetera however I've not been able to get them properly aligned.

The same does not happen with the default math font, which you can see if you compile the MWE without the two lines concerning Cambria Math.

Does anybody have any idea how to do this? Thank you all in advance.

---

EDIT: I've tried out a few of the solutions mentioned in the comments, specifically with CambriaMath as some only seem to work without it, and here are the results of the following MWE (yes, i had fun with the colors):

\documentclass{standalone}

\usepackage{unicode-math}
\setmathfont{Cambria Math}

\usepackage{tikz}

\begin{document}
\begin{tikzpicture}[every node/.style={draw, circle}]
    \draw[help lines] (-1em, 1em) grid (4cm+1em, -3cm-1em);
    \node[red, minimum width=1em] at (0, 0) {$+$};
    \node[blue, minimum width=1em] at (4, 0) {$-$};
    \node[orange, inner sep=1pt, anchor=mid] at (0, -1) {\strut$+$};
    \node[green, inner sep=1pt, anchor=mid] at (4, -1) {\strut$-$};
    \node[olive, minimum width=2em] at (0, -2) {$+$};
    \node[purple, minimum width=2em] at (4, -2) {$-$};
    \node[cyan] at (0, -3) {$+$};
    \node[magenta] at (4, -3) {$-$\vphantom{$+$}};
\end{tikzpicture}
\end{document}

Produces the following:

As can be seen in the image, the only method that seems to work with CambriaMath is the \vphantom{+} one mentioned in the comments below.

Answer 1

I think that this problem is not especially related to Cambria Math, since it seems to be rooted in the fact, that the bounding boxes of the plus and minus signs are very different and, most importantly, that of the minus sign is especially tight.

Compare for example the bounding boxes for the glyphs for Camrbia Math:

\documentclass{article}
\usepackage[margin=15mm]{geometry}
\usepackage{showcharinbox, anyfontsize}

\usepackage{unicode-math}
\setmathfont{Cambria Math}

\begin{document}
\ShowCharInBox{\fontsize{200}{210}\selectfont$+$}
\vskip 1cm
\ShowCharInBox{\fontsize{200}{210}\selectfont$-$}
\end{document}

The bounding boxes for Computer Modern on the other hand would be:

\documentclass{article}
\usepackage[margin=15mm]{geometry}
\usepackage{showcharinbox, anyfontsize}
    
\begin{document}
\ShowCharInBox{\fontsize{200}{210}\selectfont$+$}
\vskip 1cm
\ShowCharInBox{\fontsize{200}{210}\selectfont$-$}
\end{document}

(Side node: Actually, I would've guessed that the bounding boxes of the plus and minus signs are of the same height and width in Computer Modern, but obviously they're not.)

---

What to do? You can insert a \strut before or after the plus or minus sign:

\documentclass[border=10pt]{standalone}
\usepackage{unicode-math}
\setmathfont{Cambria Math}

\usepackage{tikz}

\begin{document}
\begin{tikzpicture}
    \draw[gray] (0, 0) -- (4, 0);
    \node[red, draw, circle] at (0, 0) {\strut$+$};
    \node[blue, draw, circle] at (4, 0) {\strut$-$};
\end{tikzpicture}
\end{document}

This will increase the padding a bit, but you can regulate this by setting inner sep to something small or even zero. Also, you maybe want to adjust the text depth a bit. With inner sep=1pt and text depth=0.25em, you would get:

\documentclass[border=10pt]{standalone}
\usepackage{unicode-math}
\setmathfont{Cambria Math}

\usepackage{tikz}

\begin{document}
\begin{tikzpicture}
    \draw[gray] (0, 0) -- (4, 0);
    \node[red, draw, circle, inner sep=1pt, text depth=0.25em] at (0, 0) {\strut$+$};
    \node[blue, draw, circle, inner sep=1pt, text depth=0.25em] at (4, 0) {\strut$-$};
\end{tikzpicture}
\end{document}

So what is happening is: TikZ makes the nodes as large as needed to fit each glyph, and since the height of the bounding box of the minus sign of Cambria Math is that much smaller than that of the plus sign, the glyph gets shifted upwards. Using \strut, you essentially enter a bounding box of zero width, but of maximum glyph height. Thus (and due to the fact that the width of the plus and the minus sign of Cambria Math), the two nodes become equally large.

Another way would be to insert \vphantom{$+$} as proposed by Zarko in the comments.

(Side note: Entering \strut should also work for Computer Modern as the two glyphs for plus and minus seem not to have the same height either. However, since the width of the bounding boxes of the plus and minus signs in Computer Modern are not the same, it would probably be an even better idea in this case to explicitly the size of the nodes using text width or minimum width.)