Note: The data is delayed depending on the stock exchange backing the symbol.
Clone this repository or download the finance_ file and create a symlink as root in /etc/munin/plugins by using e.g.:
cd /etc/munin/plugins; ln -s /path/to/finance_ finance_<symbol>
where is a stock symbol known to the http://finance.yahoo.com/ API like "FB" (Facebook) or "AAPL" (Apple). Use their search form to find the concrete symbol names.
Don't forget to restart your munin-node deamon.