Apologies for the late reply, we've been experiencing issues with the notification system on the Cloud forum.
If the iframe is responsive, the chart should adjust (e.g.
http://jsfiddle.net/u7c4acwj/ - try resizing the window). If this does not work for you, could you supply a link to the chart in question, as embedded on a page? If this is not possible, could you supply the CSS options for the iframe?