Hi folks,

I am having a problem making weighted projections in yt-3.  Using the current tip, when I type this:

plot = yt.ProjectionPlot(ds,'x',fields=('gas', 'density'), weight_field=None, width=(1.0,'unitary'), center=[0.5,0.5,0.5]).save()

everything works fine, and I get a projection.  However, when I try this:

plot = yt.ProjectionPlot(ds,'x',fields=('gas', 'density'), weight_field=('gas', 'density'), width=(1.0,'unitary'), center=[0.5,0.5,0.5]).save()

I get the error:

---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
<ipython-input-4-7a90d3d7dae2> in <module>()
----> 1 plot = yt.ProjectionPlot(ds,'x',fields=('gas', 'density'), weight_field=('gas', 'density'), width=(1.0,'unitary'), center=[0.5,0.5,0.5]).save()

/Users/bwoshea/Desktop/yt-3.0/src/yt-hg/yt/visualization/plot_container.pyc in save(self, name, mpl_kwargs)
    464             weight = self.data_source.weight_field
    465             if weight is not None:
--> 466                 weight = weight.replace(' ', '_')
    467         if 'Cutting' in self.data_source.__class__.__name__:
    468             type = 'OffAxisSlice'

AttributeError: 'tuple' object has no attribute 'replace'

I would note that if I use 'density' instead of ('gas','density') for the weight_field, everything works fine.  Is this the expected behavior, or is this a bug?  

Thanks!

--Brian