Remove incorrect statement about data-kwarg interface.

We don't actually require `data` to support `key in data`; we only ever
try `data[key]` (treating any exception as meaning "`key` is not in
`data`, use its value directly").  This can be checked e.g. by creating
a custom type that supports `__getitem__` but throws on `in` checks, and
verify that it can be used as `data`:
```
class T: __getitem__ = lambda self, k: {"foo": 1, "bar": 2}[k]
l, = plt.plot("foo", "bar", data=T())
l.get_xydata()  # == [[1, 2]]
"foo" in T()  # raises
```

So delete the statement about `s in data`; the statement about `data[s]`
can also be removed as it is clearly implied by the fact that "`s` is
interpreted as `data[s]`".

Author: anntzer

lib/matplotlib/__init__.py

@@ -1281,18 +1281,12 @@ def _add_data_doc(docstring, replace_names):
 
     data_doc = ("""\
     If given, all parameters also accept a string ``s``, which is
-    interpreted as ``data[s]`` (unless this raises an exception).
-
-    Objects passed as **data** must support item access (``data[s]``) and
-    membership test (``s in data``)."""
+    interpreted as ``data[s]`` (unless this raises an exception)."""
                 if replace_names is None else f"""\
-    If given, the following parameters also accept a string ``s``, which
-    is interpreted as ``data[s]`` (unless this raises an exception):
-
-    {', '.join(map('*{}*'.format, replace_names))}
+    If given, the following parameters also accept a string ``s``, which is
+    interpreted as ``data[s]`` (unless this raises an exception):
 
-    Objects passed as **data** must support item access (``data[s]``) and
-    membership test (``s in data``).""")
+    {', '.join(map('*{}*'.format, replace_names))}""")
     # using string replacement instead of formatting has the advantages
     # 1) simpler indent handling
     # 2) prevent problems with formatting characters '{', '%' in the docstring