You can create various types of charts and mathematical expressions on HackMD. Refer to the example below.
MathJax
You can render LaTeX mathematical expressions using MathJax, as on math.stackexchange.com, except the space after the start $ and the space before the end $ are not allowed in the inline math:
The Gamma function satisfying is via the Euler integral
More information about LaTeX mathematical expressions.
More information about sequence diagrams syntax here. More information about flow charts syntax here. More information about graphviz syntax here More information about mermaid syntax here More information about abc syntax here