Currently most touch users don't realise that the correct behaviour is to drag with two fingers to scroll, and one finger to zoom. As such they try to scroll the page with one finger, and find themselves zooming the chart instead.

A simple fix would be to put a "zoom" button that has to be clicked to enable the zooming mode. Ideally this would only show on touch-enabled devices, but probably be good to have the option to have it for normal mouse users too.