FIX: Nice axis bounds calculation when flipped

When the axis bounds are inverted (i.e. max < min),
the tick labels min and max were calculated incorrectly.

Change-Id: I40b95074bc6865b193a1334dd65a8356cbc13faf
FIX: Tooltips not showing up when axis inverted.

Change-Id: I102302a612facfcd6e46a88e054f1fe05bf65950
ENH: Tests for inverted axis functionality

Change-Id: I403f8db9f0c1746b45d6df38b07e98b42e1f04d8
Merge topic 'charts_invertaxis' into master

6dec986d ENH: Tests for inverted axis functionality
3fe8c432 FIX: Tooltips not showing up when axis inverted.
07b79ded FIX: Nice axis bounds calculation when flipped