Do you happen to know xypic syntax?

Oh, then presheaf will not really work for you. (Unless you're willing to spend a bit of time to learn about diagrams using xypic.)

Then I just paste it into presheaf to get this:

However, I think that basic syntax is rather simple.

You divide entries by &. \ar means arrow. You say the direction by [d] = down, [r] =right and similar.

Images are entered simply by clicking on upload. But I see you have already realized that.

BTW I see that I have in fact drawn a nonsensical diagram.

To include a very simple example, if I just enter A & B I get A and B diagram between them. I can add arrow \ar[r] at A which goes to the right (from A to B). And I can add some caption above or below the arrow.

Done by: A \ar[r]_f & B. presheaf.com/?d=d2j254b3t3n1d6d4c2a22594b4a2b1746

There is much more stuff you can do with xypic. But basics are pretty easy.

Here is a fancier example: A \ar@<0.5ex>[r]^f \ar@<-0.5ex>[r]_g & B

Yes, there are several manuals for xypic on the internet. Usually, for simply stuff I am able to do without. And if I need something more complicated, I either look into help or copy paste from somewhere (the latter mainly if I drew some similar diagrams in the past).

As you probably know, some people use AMScd. This is supported by MathJax. But no diagonal arrow.

Meta: How to draw a commutative diagram?

@MartinSleziak @user170039 I recently found this tool for drawing diagrams, definitely worth trying out: tikzcd.yichuanshen.de

It seems to be the same link as posted here: chat.stackexchange.com/transcript/message/41044564#41044564 (just without obscuring).

whoops, didn't pay attention :)

It's probably better to have direct link.

I have heard that tikz is a useful package for drawing diagrams in general. As I have a lot on my plate at the moment, I am probably not going to start learning a new TeX package at the moment.

For commutative diagrams xypic is entirely sufficient for me. For drawing pictures in general, I tend to use metapost.

Sure, the nice thing about yi's website is that you don't need to know any package

Judging by the result on TeX.SE, tikz seems to be rather popular: TeX Community Polls.

I haven't dabbled a lot with tikz, because you need to specify all the coordinates manually, at least I understood it that way

so I guess xypic is more in the spirit of latex ^^