Zoom does not zoom out when on Zoom Limits

[GoogleCodeExporter]

Try to reproduce:
1. Open a demo with zoom limits set, lika Average Temperature
2. Zoom In a lot
3. Pan to the bottom of the area (about 1 hour, on right vertical axe)
4. Try to Zoom out (with pinchZoom or zoom button)
5. Vertical zoom does not change, because we are at zoom limit

The expected behavior is that the zoom level change but the limits is kept, so 
only area from upper part of chart enter in the visible windows.

With achartengine 1.2

Original issue reported on code.google.com by arlab....@gmail.com on 27 Mar 2015 at 3:52