Questions: arrays, and diagrams

[Alasdair McAndrew]

Arrays. I know that Mathjax only supports a subset of the LaTeX array environment parameters.  I would like to squash up an array horizontally - which in LaTeX I would do by setting the \arraycolsep variable to be small.  How can I do this with Mathjax?  Another array issue is that the \begin{array} ... \end{array} pair automatically trigger Mathjax, so putting surrounding LaTeX delimiters like \[  .. \] or $$ ... $$ is unnecessary.  In fact , such delimiters get rendered as part of the document text.  But if I want to include several arrays together, such as a matrix equation

\begin{bmatrix}1&2\\ 3&4\end{bmatrix}  \begin{bmatrix}x\\ y \end{bmatrix} = \begin{bmatrix}5\\ 6 \end{bmatrix} 

I would want to put them in a set of equation delimiters to render the entire lot as a single mathematical expression.  How should I do this?

Diagrams. As Mathjax doesn't support diagrams, I'm wondering what people here use to create mathematical diagrams for display online?

Thank you,

Alasdair