Zoom out with mouse
In the same way that you zoom in on a chart by rubber-banding with the mouse by clicking and dragging from top left to bottom right add functionality to allow zooming out (to full extents) by doing the reverse - i.e. by clicking bottom right and dragging to top left. Very simple - you could then ditch the reset zoom button if you wanted.
Dan Dascalescu commented
There is a plugin for this that lets you zoom out by dragging right to left: