Combination of newtx and fontspec breaks siunitx

Question

I am trying to use newtx and fontspec on XeLaTeX, and it looked OK until I used siunitx. The problem is negative signs are not printed by \num etc. How can I fix this?

Here is MWE:

\documentclass{article}
\usepackage[no-math]{fontspec}
\usepackage{zxjatype} % For printing Japanese. Requires fontspec.
\usepackage[ipaex]{zxjafont} % Set Japanese font.
\usepackage{newtxtext,newtxmath}
\usepackage{siunitx}
\sisetup{
  detect-family=true,
  detect-weight=true,
  detect-mode=true,
  detect-inline-family=math,
  detect-inline-weight=math,
  detect-display-math=true,
  output-quotient=\ensuremath{/},
}
\begin{document}
\textminus % OK

\num{-1e-1} % negative sign is not printed
\end{document}

Though my problem can be reproduced without zxjatype and zxjafont, I included them in the above example because it may be also affect my problem.

Background
I want to create a plot to include in my pLaTeX (Japanese LaTeX) document with a Python library matplotlib, which allows you to typeset text with XeLaTeX but not pLaTeX. Because XeLaTeX can also typeset Japanese text, I decided to use XeLaTeX to typeset Japanese in matolotlib. Because my main document is written in pLaTeX, which has better support on Japanese but does not support fontspec, I want to use newtxtext on both engines. I do not want to use TeX Gyre Termes or Times on fontspec because some glyphs have very different shapes compared to newtx (e.g. \textmu).

Answer 1

Well you can reset the minus sign:

\documentclass{article}
\usepackage[no-math]{fontspec}
\usepackage{newtxtext,newtxmath}
\usepackage{siunitx}
\sisetup{
  detect-family=true,
  detect-weight=true,
  detect-mode=true,
  detect-inline-family=math,
  detect-inline-weight=math,
  detect-display-math=true,
  output-quotient=\ensuremath{/},
}

\ExplSyntaxOn
\AtBeginDocument{
\tl_set:Nn \c__siunitx_minus_tl
            { \textminus }
        }
\ExplSyntaxOff

\begin{document}
\num{-1e-1} % negative sign is not printed
\end{document}

But imho it is a rather bad idea to use T1-encoding as main encoding with xelatex. Quite probably more symbols will "disappear".

Answer 2

You shouldn't use newtx, but rather a Times-like text font

\documentclass{article}
\usepackage{newtxmath}
\usepackage[no-math]{fontspec}
\usepackage{zxjatype} % For printing Japanese. Requires fontspec.
\usepackage[ipaex]{zxjafont} % Set Japanese font.
\setmainfont{TeX Gyre Termes}
\usepackage{siunitx}

% see http://tex.stackexchange.com/a/279466/4427    
\DeclareSymbolFont{operators}{\encodingdefault}{\familydefault}{m}{n}

\sisetup{
  detect-family=true,
  detect-weight=true,
  detect-mode=true,
  detect-inline-family=math,
  detect-inline-weight=math,
  detect-display-math=true,
  output-quotient=\ensuremath{/},
}
\begin{document}

\textminus % OK

\num{-1e-1}

$\sin(\pi/3)\approx 0.866$

\end{document}