Tooltips do not show up if chart is within a container with overflow auto and a fixed height

This issue happened when using the mootools adapter. Don't really know if this happens for others. Here's a [fiddle](http://jsfiddle.net/U8Dwf/3/) so you can quickly reproduce the bug.

The main problem lies in the adapter. It was calling an undocumented `getOffsets()` mootools function. This function does not account for the scroll position, causing wrong calculations when the chart is deep within the overflowed container.