Support lon/lat bounds in QuadMesh

pcolormesh will work correctly for 2D data with bounds according to the definition.

I tested the Dataset below:

<xarray.Dataset>
Dimensions:                              (y_bounds: 4173, x_bounds: 451, y: 4172, x: 450)
Coordinates:
    lon_bounds                           (y_bounds, x_bounds) float32 dask.array<chunksize=(4096, 451), meta=np.ndarray>
    lat_bounds                           (y_bounds, x_bounds) float32 dask.array<chunksize=(4096, 451), meta=np.ndarray>
    time                                 datetime64[ns] 2019-08-10
    latitude                             (y, x) float32 dask.array<chunksize=(4096, 450), meta=np.ndarray>
    longitude                            (y, x) float32 dask.array<chunksize=(4096, 450), meta=np.ndarray>
    crs                                  object +proj=latlong +datum=WGS84 +e...
Dimensions without coordinates: y_bounds, x_bounds, y, x
Data variables:
    nitrogendioxide_tropospheric_column  (y, x) float32 dask.array<chunksize=(4096, 450), meta=np.ndarray>

import geoviews as gv

gvds = gv.Dataset(ds.unify_chunks()).persist()

mesh = gvds.to(gv.QuadMesh,
        kdims=["lon_bounds", "lat_bounds"],
        vdims=['nitrogendioxide_tropospheric_column'])

Here’s the error:

---------------------------------------------------------------------------
KeyError                                  Traceback (most recent call last)
/tmp/ipykernel_163767/1539571452.py in <module>
      2 gvds = gv.Dataset(ds.unify_chunks()).persist()
      3 
----> 4 mesh = gvds.to(gv.QuadMesh,
      5         kdims=["lon_bounds", "lat_bounds"],
      6         vdims=['nitrogendioxide_tropospheric_column'])

~/new/miniconda3/envs/arctic/lib/python3.8/site-packages/geoviews/element/__init__.py in __call__(self, *args, **kwargs)
     37                 args = (Dataset,)
     38                 kwargs['kdims'] = []
---> 39         converted = super(GeoConversion, self).__call__(*args, **kwargs)
     40         if is_gpd:
     41             if kdims is None: kdims = group_type.kdims

~/new/miniconda3/envs/arctic/lib/python3.8/site-packages/holoviews/core/data/__init__.py in __call__(self, new_type, kdims, vdims, groupby, sort, **kwargs)
    142             else:
    143                 selected = self._element.reindex(groupby+kdims, vdims)
--> 144         params = {'kdims': [selected.get_dimension(kd, strict=True) for kd in kdims],
    145                   'vdims': [selected.get_dimension(vd, strict=True) for vd in vdims],
    146                   'label': selected.label}

~/new/miniconda3/envs/arctic/lib/python3.8/site-packages/holoviews/core/data/__init__.py in <listcomp>(.0)
    142             else:
    143                 selected = self._element.reindex(groupby+kdims, vdims)
--> 144         params = {'kdims': [selected.get_dimension(kd, strict=True) for kd in kdims],
    145                   'vdims': [selected.get_dimension(vd, strict=True) for vd in vdims],
    146                   'label': selected.label}

~/new/miniconda3/envs/arctic/lib/python3.8/site-packages/holoviews/core/dimension.py in get_dimension(self, dimension, default, strict)
    987             name_map.update({util.dimension_sanitizer(dim.name): dim for dim in all_dims})
    988             if strict and dimension not in name_map:
--> 989                 raise KeyError("Dimension %r not found." % dimension)
    990             else:
    991                 return name_map.get(dimension, default)

KeyError: "Dimension 'lon_bounds' not found."

So, I suppose we can’t use bounds at this stage?

Note that if we don’t use bounds for pcolormesh, it will produce strange stripes over the polar region like this:

[maximlt]

Great thanks @zxdawn for reporting how to do that correctly 😃

[zxdawn]

@maximlt You’re welcome. I found another problem of setting the extent of the project and posted it on the forum. If that’s actually an issue, I can submit it on GitHub. Otherwise, if that’s a usage topic, I will keep it there.