[yt-dev] Issue #1242: SlicePlot & ProjectionPlot show all zero field (yt_analysis/yt)

[Bili Dong]

New issue 1242: SlicePlot & ProjectionPlot show all zero field
https://bitbucket.org/yt_analysis/yt/issues/1242/sliceplot-projectionplot-show-all-zero

Bili Dong:

I just updated to the development tip (3.3_dev-py27_2755 as shown in conda), and suddenly the SlicePlot (and similarly the ProjectionPlot) doesn't work properly. When I run the following code:

```
#!python

import yt 

ds = yt.load('FIRE_M12i_ref11/snapshot_600.hdf5')
dmax, dcenter = ds.find_max(('PartType0', 'Density'))
p = yt.SlicePlot(ds, 'z', ('gas', 'density'), center=dcenter, width=(1, 'Mpc'))
p.save()
```

I get the following plot:
![snapshot_600_Slice_z_density.png](https://bitbucket.org/repo/BXbAb/images/799920682-snapshot_600_Slice_z_density.png)

The code used to work with previous version of yt, so I'm wondering if it's only me or there is a bug.