Validate plot backend when setting.

```python
In [1]: import pandas as pd

In [2]: import sys

In [3]: import types

In [4]: module = types.ModuleType("foo")

In [5]: sys.modules['foo'] = module

In [6]: pd.set_option('plotting.backend', 'foo')
---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
<ipython-input-6-123b6513deb2> in <module>
----> 1 pd.set_option('plotting.backend', 'foo')

...

~/sandbox/pandas/pandas/plotting/_core.py in _find_backend(backend)
   1588         "top-level `.plot` method."
   1589     )
-> 1590     raise ValueError(msg.format(name=backend))
   1591
   1592

ValueError: Could not find plotting backend 'foo'. Ensure that you've installed the package providing the 'foo' entrypoint, or that the package has atop-level `.plot` method.

```

Closes pandas-dev#28163

Author: TomAugspurger

doc/source/whatsnew/v1.0.0.rst

@@ -171,6 +171,7 @@ Plotting
 - Bug in :meth:`DataFrame.plot` producing incorrect legend markers when plotting multiple series on the same axis (:issue:`18222`)
 - Bug in :meth:`DataFrame.plot` when ``kind='box'`` and data contains datetime or timedelta data. These types are now automatically dropped (:issue:`22799`)
 - Bug in :meth:`DataFrame.plot.line` and :meth:`DataFrame.plot.area` produce wrong xlim in x-axis (:issue:`27686`, :issue:`25160`, :issue:`24784`)
+- :func:`set_option` now validates that the plot backend provided to ``'plotting.backend'`` implements the backend when the option is set, rather than when a plot is created (:issue:`28163`)
 
 Groupby/resample/rolling
 ^^^^^^^^^^^^^^^^^^^^^^^^

pandas/core/config_init.py

@@ -9,8 +9,6 @@
 module is imported, register them here rather then in the module.
 
 """
-import importlib
-
 import pandas._config.config as cf
 from pandas._config.config import (
     is_bool,
@@ -581,26 +579,10 @@ def use_inf_as_na_cb(key):
 
 
 def register_plotting_backend_cb(key):
-    backend_str = cf.get_option(key)
-    if backend_str == "matplotlib":
-        try:
-            import pandas.plotting._matplotlib  # noqa
-        except ImportError:
-            raise ImportError(
-                "matplotlib is required for plotting when the "
-                'default backend "matplotlib" is selected.'
-            )
-        else:
-            return
+    from pandas.plotting._core import _get_plot_backend
 
-    try:
-        importlib.import_module(backend_str)
-    except ImportError:
-        raise ValueError(
-            '"{}" does not seem to be an installed module. '
-            "A pandas plotting backend must be a module that "
-            "can be imported".format(backend_str)
-        )
+    backend_str = cf.get_option(key)
+    _get_plot_backend(backend_str)
 
 
 with cf.config_prefix("plotting"):

pandas/plotting/_core.py

@@ -1576,10 +1576,18 @@ def _find_backend(backend: str):
             # We re-raise later on.
             pass
         else:
-            _backends[backend] = module
-            return module
-
-    raise ValueError("No backend {}".format(backend))
+            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):
@@ -1600,7 +1608,13 @@ def _get_plot_backend(backend=None):
     if backend == "matplotlib":
         # Because matplotlib is an optional dependency and first-party backend,
         # we need to attempt an import here to raise an ImportError if needed.
-        import pandas.plotting._matplotlib as module
+        try:
+            import pandas.plotting._matplotlib as module
+        except ImportError:
+            raise ImportError(
+                "matplotlib is required for plotting when the "
+                "default backend 'matplotlib' is selected."
+            ) from None
 
         _backends["matplotlib"] = module
 

pandas/tests/plotting/test_backend.py

@@ -9,6 +9,17 @@
 import pandas
 
 
+@pytest.fixture
+def dummy_backend():
+    backend = types.ModuleType("pandas_dummy_backend")
+    backend.plot = lambda *args, **kwargs: None
+    sys.modules["pandas_dummy_backend"] = backend
+
+    yield
+
+    del sys.modules["pandas_dummy_backend"]
+
+
 def test_matplotlib_backend_error():
     msg = (
         "matplotlib is required for plotting when the default backend "
@@ -22,20 +33,14 @@ def test_matplotlib_backend_error():
 
 
 def test_backend_is_not_module():
-    msg = (
-        '"not_an_existing_module" does not seem to be an installed module. '
-        "A pandas plotting backend must be a module that can be imported"
-    )
+    msg = "Could not find plotting backend 'not_an_existing_module'."
     with pytest.raises(ValueError, match=msg):
         pandas.set_option("plotting.backend", "not_an_existing_module")
 
 
-def test_backend_is_correct(monkeypatch):
-    monkeypatch.setattr(
-        "pandas.core.config_init.importlib.import_module", lambda name: None
-    )
-    pandas.set_option("plotting.backend", "correct_backend")
-    assert pandas.get_option("plotting.backend") == "correct_backend"
+def test_backend_is_correct(dummy_backend):
+    pandas.set_option("plotting.backend", "pandas_dummy_backend")
+    assert pandas.get_option("plotting.backend") == "pandas_dummy_backend"
 
     # Restore backend for other tests (matplotlib can be not installed)
     try:
@@ -74,6 +79,16 @@ def test_register_entrypoint():
     assert result is mod
 
 
+def test_setting_backend_raies():
+    module = types.ModuleType("pandas_plot_backend")
+    sys.modules["pandas_plot_backend"] = module
+
+    with pytest.raises(
+        ValueError, match="Could not find plotting backend 'pandas_plot_backend'."
+    ):
+        pandas.set_option("plotting.backend", "pandas_plot_backend")
+
+
 def test_register_import():
     mod = types.ModuleType("my_backend2")
     mod.plot = lambda *args, **kwargs: 1