Yun has charts built in. Some people have used it. It is part of the original Openwrt system.
Small tip: do not call any of the php-files something with data.php or datasensor.php as the word 'data' messes the Yun up and it can't cope (spent two evenings trying to find out what I was doing wrong, turned out to be the filename)
I wonder if any of those also trigger your problem?