Add entrypoint for plotting backends

ci/requirements/py37.yml

@@ -17,6 +17,7 @@ dependencies:
   - h5netcdf
   - h5py
   - hdf5
+  - hvplot
   - hypothesis
   - iris
   - isort

setup.cfg

@@ -90,6 +90,10 @@ xarray =
     static/css/*
     static/html/*
 
+[options.entry_points]
+xarray_plotting_backends = 
+    matplotlib = xarray:plot
+
 [tool:pytest]
 python_files = test_*.py
 testpaths = xarray/tests properties
@@ -197,4 +201,4 @@ ignore_errors = True
 test = pytest
 
 [pytest-watch]
-nobeep = True
+nobeep = True

xarray/core/options.py

@@ -9,6 +9,7 @@
 CMAP_DIVERGENT = "cmap_divergent"
 KEEP_ATTRS = "keep_attrs"
 DISPLAY_STYLE = "display_style"
+PLOTTING_BACKEND = "plotting_backend"
 
 
 OPTIONS = {
@@ -21,6 +22,7 @@
     CMAP_DIVERGENT: "RdBu_r",
     KEEP_ATTRS: "default",
     DISPLAY_STYLE: "text",
+    PLOTTING_BACKEND: "matplotlib",
 }
 
 _JOIN_OPTIONS = frozenset(["inner", "outer", "left", "right", "exact"])
@@ -39,6 +41,7 @@ def _positive_integer(value):
     WARN_FOR_UNCLOSED_FILES: lambda value: isinstance(value, bool),
     KEEP_ATTRS: lambda choice: choice in [True, False, "default"],
     DISPLAY_STYLE: _DISPLAY_OPTIONS.__contains__,
+    PLOTTING_BACKEND: lambda value: isinstance(value, str),
 }
 
 
@@ -56,9 +59,16 @@ def _warn_on_setting_enable_cftimeindex(enable_cftimeindex):
     )
 
 
+def _set_plotting_backend(backend):
+    from ..plot.utils import _get_plot_backend
+
+    return _get_plot_backend(backend)
+
+
 _SETTERS = {
     FILE_CACHE_MAXSIZE: _set_file_cache_maxsize,
     ENABLE_CFTIMEINDEX: _warn_on_setting_enable_cftimeindex,
+    PLOTTING_BACKEND: _set_plotting_backend,
 }
 
 
@@ -104,6 +114,10 @@ class set_options:
       Default: ``'default'``.
     - ``display_style``: display style to use in jupyter for xarray objects.
       Default: ``'text'``. Other options are ``'html'``.
+    - ``plotting_backend``: The name of plotting backend to use. Backends can be implemented
+      as third-party libraries implementing the xarray plotting API. They can use other
+      plotting libraries like Bokeh, Holoviews, Hvplot, Altair, etc.
+      Default: ``'matplotlib'``
 
 
     You can use ``set_options`` either as a context manager:

xarray/plot/utils.py

@@ -575,7 +575,6 @@ def _add_colorbar(primitive, ax, cbar_ax, cbar_kwargs, cmap_params):
         cbar_kwargs.setdefault("ax", ax)
     else:
         cbar_kwargs.setdefault("cax", cbar_ax)
-
     cbar = plt.colorbar(primitive, **cbar_kwargs)
 
     return cbar
@@ -772,3 +771,87 @@ def _process_cmap_cbar_kwargs(
     cmap_params = _determine_cmap_params(**cmap_kwargs)
 
     return cmap_params, cbar_kwargs
+
+
+_backends = {}
+
+
+def _find_backend(backend):
+    """
+    Find an xarray plotting backend
+
+    Parameters
+    ----------
+    backend : str
+        The identifier for the backend. Either an entrypoint item registered
+        with pkg_resources, or a module name.
+
+    Notes
+    -----
+    Modifies _backends with imported backends as a side effect.
+
+    Returns
+    -------
+    types.ModuleType
+        The imported backend.
+    """
+
+    import pkg_resources  # Delay import for performance.
+    import importlib
+
+    for entry_point in pkg_resources.iter_entry_points("xarray_plotting_backends"):
+        if entry_point.name == "matplotlib":
+            # matplotlib is an optional dependency. When
+            # missing, this would raise.
+            continue
+        _backends[entry_point.name] = entry_point.load()
+
+    try:
+        return _backends[backend]
+    except KeyError:
+        # Fall back to unregisted, module name approach.
+        try:
+            module = importlib.import_module(backend)
+        except ImportError:
+            # We re-raise later on.
+            pass
+
+        else:
+            if hasattr(module, "plot"):
+                # Validate that the interface is implemented when the option is set,
+                # rather than at plot time
+                _backends[backend] = module
+                return module
+    msg = (
+        "Could not find plotting backend '{name}'. Ensure that you've installed the "
+        "package providing the '{name}' entrypoint, or that the package has a"
+        "top-level `.plot` method."
+    )
+
+    raise ValueError(msg.format(name=backend))
+
+
+def _get_plot_backend(backend=None):
+    """
+    Return the plotting backend to use
+    """
+
+    backend = backend or OPTIONS["plotting_backend"]
+
+    if backend == "matplotlib":
+        try:
+            import xarray.plot as module
+        except ImportError:
+            raise ImportError(
+                "matplotlib is required for plotting when the "
+                'default backend "matplotlib" is selected.'
+            ) from None
+
+        _backends["matplotlib"] = module
+
+    if backend in _backends:
+        return _backends[backend]
+
+    module = _find_backend(backend)
+    _backends[backend] = module
+    return module

xarray/tests/__init__.py

@@ -85,6 +85,8 @@ def LooseVersion(vstring):
     has_seaborn = False
 requires_seaborn = pytest.mark.skipif(not has_seaborn, reason="requires seaborn")
 
+has_hvplot, requires_hvplot = _importorskip("hvplot")
+
 # change some global options for tests
 set_options(warn_for_unclosed_files=True)
 

xarray/tests/test_options.py

@@ -77,6 +77,18 @@ def test_display_style():
     assert OPTIONS["display_style"] == original
 
 
+def test_plotting_backend():
+    original = "matplotlib"
+    assert OPTIONS["plotting_backend"] == original
+    with pytest.raises(ValueError):
+        xarray.set_options(plotting_backend=5)
+
+    with xarray.set_options(plotting_backend="holoviews"):
+        assert OPTIONS["plotting_backend"] == "holoviews"
+
+    assert OPTIONS["plotting_backend"] == original
+
+
 def create_test_dataset_attrs(seed=0):
     ds = create_test_data(seed)
     ds.attrs = {"attr1": 5, "attr2": "history", "attr3": {"nested": "more_info"}}

xarray/tests/test_plot.py

@@ -24,6 +24,7 @@
     has_nc_time_axis,
     raises_regex,
     requires_cftime,
+    requires_hvplot,
     requires_matplotlib,
     requires_nc_time_axis,
     requires_seaborn,
@@ -2228,3 +2229,13 @@ def test_plot_transposes_properly(plotfunc):
     # pcolormesh returns 1D array but imshow returns a 2D array so it is necessary
     # to ravel() on the LHS
     assert np.all(hdl.get_array().ravel() == da.to_masked_array().ravel())
+
+
+@requires_matplotlib
+@requires_hvplot
+@pytest.mark.parametrize("plotting_backend", ["matplotlib", "hvplot.plotting"])
+def test_plotting_backend(plotting_backend):
+    air = xr.tutorial.open_dataset("air_temperature").load().air
+
+    with xr.set_options(plotting_backend=plotting_backend):
+        air.isel(time=500).plot(add_colorbar=False)