(update) Just in case anyone else is interested in this, here's a combination I just found that comes close to being ideal, for my taste.
Similar math font as that of the mathdesign package, but slightly 'lighter', and spaced more liberally.
The settings are an adjusted version of Example 2 of XCharter documentation. All credit goes to @samcarter for pointing this out.
%% main settings
% scaled to match Charter font size of mathdesign, which I like best
% [osf] reduces number size in text mode... not sure yet about this one
\usepackage[scale=0.97]{XCharter}
% scale math size to taste
\usepackage[libertine,bigdelims,vvarbb,scaled=1.05]{newtxmath}
% error fix
\let\openbox\undefined
%% optional settings
% I have no idea what I'm doing here.jpeg, but it sure looks prettier
\usepackage[spacing=true,kerning=true,babel=true,tracking=true]{microtype}
% different \mathcal style, but just a matter of taste
\DeclareMathAlphabet{\pazocal}{OMS}{zplm}{m}{n}
\renewcommand{\mathcal}[1]{\pazocal{#1}}
I am currently using
\usepackage[charter]{mathdesign}
While I like the text font, the math is spaced too densely for my taste.
I tried as an alternative
\usepackage[scale=0.97]{XCharter}
which seems to give me nearly identical text, but allows to set a different font for math mode.
The default math (which I get when using XCharter) is spaced the way I want it, but the font looks too thin/not heavy enough next to the Charter text font.
Alternatively, I tried
\usepackage[T1]{eulervm}
which is spaced nicely and looks 'heavy' enough, but I really don't like the characters (glyphs?).
(edit) Ideally, I would like something very similar to the default math font+spacing, but with a heavier font.
As you can probably tell, I'm not really knowledgeable about the tex internals here. For example, I don't know what controls the spacing in math mode.
So my question consists of two parts:
What would be a good "math font" to go with Charter text font (in the wide sense above, and probably not the right name for what I want, which is "math fonts + spacing")
How can I set it independently of the text fonts?
(Apologies if the following question is too "open ended", i.e. if "What would look good with X" isn't the type of question one should ask on this SE)
\usepackage[scale=0.97]{XCharter}, will I load the math font by another\usepackage? or do I need to use something like\setmathfont, which I still struggle with? – Bert Zangle Jun 26 '17 at 15:13\usepackage{arev}for math and\usepackage{XCharter}for text. That doesn't seem to work though, and all fonts are from whatever is loaded last. Any pointer to an older question on SE showing how to fix that? – Bert Zangle Jun 26 '17 at 15:30\renewcommand{\rmdefault}{mdbch}[this will change the normal text to charter and leave the previously chosen math alone] – samcarter_is_at_topanswers.xyz Jun 26 '17 at 15:31