SciTools/iris — pull request #97 by esc24 (2012-09-18 12:40:14+00:00)
Title: Axis unit labels

Whilst tidying up the derived phenomena example I noticed what appeared to be an unnecessary assignment to the derived cube's units. It was already in kelvin (as calculated by the product of two cubes), but there was an explicit assignment of units i.e.
```python
air_temperature = exner_pressure * air_potential_temperature
# set phenomenon definition and unit
air_temperature.standard_name = 'air_temperature'
air_temperature.units = 'K'
```
Removing this leaves the unit as ```iris.unit.Unit('kelvin')``` since:
```python
>>> iris.unit.Unit('K') * iris.unit.Unit('1')
Unit('kelvin')
```
so the assignment appears to be there to change the label not the physical unit. I believe this should not be used in an example:
1. it implies Iris does not propagate the unit
2. it shows assignment to units without caution regarding the existing unit and the values of the data

This PR modifies quickplot axis labelling to remove '1' e.g. when plotting ```model level number``` one now gets 'Model level number' rather than 'Model level number / 1', and makes use of ```iris.unit.Unit.symbol``` when shorter than the full unit string e.g. to avoid 'kelvin' replacing 'K'. One downside is the abbreviation of units such as 'days' to 'd'.