Plots using an integer axis produces float labels

For example,
```python
arr = ndtest('year=2010..2012')
arr.plot()
```
produces a plot from 2010.0 to 2012.0 in steps of 0.25, which looks really silly. Remarkably, the plot produced via the editor ***GUI*** (via Ctrl-P) does not suffer from that problem, so its code could probably be used for inspiration.