New zoom mode that scales horizontally and vertically on mouse movement

[kangalio]

I'm considering porting an Python pyqtgraph application to JavaScript using Chart.js. Pyqtgraph supports not just panning with the left mouse button and zooming with the mouse wheel, but also scaling horizontally and vertically with the right mouse button (link). This turns out to be really intuitive and fast for exploring the data in my application.

Could this feature be added to Chart.js's zoom plugin too?

Instead of right-click dragging, we'd probably have left-click + modifier key dragging, since Hammer.js apparently doesn't support right-click dragging, and also since it fits in better with the existing mouse interactions in this plugin.

The options entry could be called scale

Would such a PR be accepted?