Removals

Stricter validation of function parameters
..........................................

- Calling `.Figure.add_axes` with no arguments will raise an error. Adding a
  free-floating axes needs a position rectangle. If you want a figure-filling
  single axes, use `.Figure.add_subplot` instead.
- `.Figure.add_subplot` validates its inputs; in particular, for
  ``add_subplot(rows, cols, index)``, all parameters must be integral.
  Previously strings and floats were accepted and converted to int.
- Passing *None* as the *which* argument to ``autofmt_xdate`` is no longer
  supported; use its more explicit synonym, ``which="major"``, instead.
- Setting the *orientation* of an ``eventplot()`` or `.EventCollection` to
  "none" or *None* is no longer supported; set it to "horizontal" instead.
  Moreover, the two orientations ("horizontal" and "vertical") are now
  case-sensitive.
- Passing parameters *norm* and *vmin*/*vmax* simultaneously to functions using
  colormapping such as ``scatter()`` and ``imshow()`` is no longer supported.
  Instead of ``norm=LogNorm(), vmin=min_val, vmax=max_val`` pass
  ``norm=LogNorm(min_val, max_val)``. *vmin* and *vmax* should only be used
  without setting *norm*.
- Passing *None* as either the *radius* or *startangle* arguments of an
  `.Axes.pie` is no longer accepted; use the explicit defaults of 1 and 0,
  respectively, instead.
- Passing *None* as the *normalize* argument of `.Axes.pie` (the former
  default) is no longer accepted, and the pie will always be normalized by
  default. If you wish to plot an incomplete pie, explicitly pass
  ``normalize=False``.
- Support for passing *None* to ``subplot_class_factory`` has been removed.
  Explicitly pass in the base `~matplotlib.axes.Axes` class instead.
- Passing multiple keys as a single comma-separated string or multiple
  arguments to `.ToolManager.update_keymap` is no longer supported; pass keys
  as a list of strings instead.
- Passing the dash offset as *None* is no longer accepted, as this was never
  universally implemented, e.g. for vector output. Set the offset to 0 instead.
- Setting a custom method overriding `.Artist.contains` using
  ``Artist.set_contains`` has been removed, as has ``Artist.get_contains``.
  There is no replacement, but you may still customize pick events using
  `.Artist.set_picker`.
- `~.Axes.semilogx`, `~.Axes.semilogy`, `~.Axes.loglog`, `.LogScale`, and
  `.SymmetricalLogScale` used to take keyword arguments that depends on the
  axis orientation ("basex" vs "basey", "subsx" vs "subsy", "nonposx" vs
  "nonposy"); these parameter names have been removed in favor of "base",
  "subs", "nonpositive". This removal also affects e.g. ``ax.set_yscale("log",
  basey=...)`` which must now be spelled ``ax.set_yscale("log", base=...)``.

  The change from "nonpos" to "nonpositive" also affects
  `~.scale.LogTransform`, `~.scale.InvertedLogTransform`,
  `~.scale.SymmetricalLogTransform`, etc.

  To use *different* bases for the x-axis and y-axis of a `~.Axes.loglog` plot,
  use e.g. ``ax.set_xscale("log", base=10); ax.set_yscale("log", base=2)``.
- Passing *None*, or no argument, to ``parasite_axes_class_factory``,
  ``parasite_axes_auxtrans_class_factory``, ``host_axes_class_factory`` is no
  longer accepted; pass an explicit base class instead.

Case-sensitivity is now enforced more
......................................

- Upper or mixed-case property names are no longer normalized to lowercase in
  `.Artist.set` and `.Artist.update`. This allows one to pass names such as
  *patchA* or *UVC*.
- Case-insensitive capstyles and joinstyles are no longer lower-cased; please
  pass capstyles ("miter", "round", "bevel") and joinstyles ("butt", "round",
  "projecting") as lowercase.
- Saving metadata in PDF with the PGF backend no longer changes keys to
  lowercase. Only the canonically cased keys listed in the PDF specification
  (and the `~.backend_pgf.PdfPages` documentation) are accepted.

No implicit initialization of ``Tick`` attributes
.................................................

The `.Tick` constructor no longer initializes the attributes ``tick1line``,
``tick2line``, ``gridline``, ``label1``, and ``label2`` via ``_get_tick1line``,
``_get_tick2line``, ``_get_gridline``, ``_get_text1``, and ``_get_text2``.
Please directly set the attribute in the subclass' ``__init__`` instead.

``NavigationToolbar2`` subclass changes
.......................................

Overriding the ``_init_toolbar`` method of `.NavigationToolbar2` to initialize
third-party toolbars is no longer supported. Instead, the toolbar should be
initialized in the ``__init__`` method of the subclass (which should call the
base-class' ``__init__`` as appropriate).

The ``press`` and ``release`` methods of `.NavigationToolbar2` were called when
pressing or releasing a mouse button, but *only* when an interactive pan or
zoom was occurring (contrary to what the docs stated). They are no longer
called; if you write a backend which needs to customize such events, please
directly override ``press_pan``/``press_zoom``/``release_pan``/``release_zoom``
instead.

Removal of old file mode flag
.............................

Flags containing "U" passed to `.cbook.to_filehandle` and `.cbook.open_file_cm`
are no longer accepted. This is consistent with their removal from `open` in
Python 3.9.

Keymaps toggling ``Axes.get_navigate`` have been removed
........................................................

This includes numeric key events and rcParams.

The ``TTFPATH`` and ``AFMPATH`` environment variables
.....................................................

Support for the (undocumented) ``TTFPATH`` and ``AFMPATH`` environment
variables has been removed. Register additional fonts using
``matplotlib.font_manager.fontManager.addfont()``.

Modules
~~~~~~~

- ``matplotlib.backends.qt_editor.formsubplottool``; use
  ``matplotlib.backends.backend_qt.SubplotToolQt`` instead.
- ``matplotlib.compat``
- ``matplotlib.ttconv``
- The Qt4-based backends, ``qt4agg`` and ``qt4cairo``, have been removed. Qt4
  has reached its end-of-life in 2015 and there are no releases of either PyQt4
  or PySide for recent versions of Python. Please use one of the Qt5 or Qt6
  backends.

Classes, methods and attributes

- ``bezier.make_path_regular`` has been removed; use ``Path.cleaned()`` (or
  ``Path.cleaned(curves=True)``, etc.) instead, but note that these methods add
  a ``STOP`` code at the end of the path.
- ``bezier.concatenate_paths`` has been removed; use
  ``Path.make_compound_path()`` instead.
- ``cbook.local_over_kwdict`` has been removed; use `.cbook.normalize_kwargs`
  instead.
- ``qt_compat.is_pyqt5`` has been removed due to the release of PyQt6. The Qt
  version can be checked using ``QtCore.qVersion()``.
- ``testing.compare.make_external_conversion_command`` has been removed.
- ``axes_grid1.axes_rgb.imshow_rgb`` has been removed; use
  ``imshow(np.dstack([r, g, b]))`` instead.

Arguments

- The ``animation.avconv_path`` and ``animation.avconv_args`` rcParams have
  been removed.
- The ``animation.html_args`` rcParam has been removed.
- The ``keymap.all_axes`` rcParam has been removed.
- The ``mathtext.fallback_to_cm`` rcParam has been removed. Use
  :rc:`mathtext.fallback` instead.
- The ``savefig.jpeg_quality`` rcParam has been removed.
- The ``text.latex.preview`` rcParam has been removed.
- The following deprecated rcParams validators, defined in `.rcsetup`, have
  been removed:

  - ``validate_alignment``
  - ``validate_axes_titlelocation``
  - ``validate_axis_locator``
  - ``validate_bool_maybe_none``
  - ``validate_fontset``
  - ``validate_grid_axis``
  - ``validate_hinting``
  - ``validate_legend_loc``
  - ``validate_mathtext_default``
  - ``validate_movie_frame_fmt``
  - ``validate_movie_html_fmt``
  - ``validate_movie_writer``
  - ``validate_nseq_float``
  - ``validate_nseq_int``
  - ``validate_orientation``
  - ``validate_pgf_texsystem``
  - ``validate_ps_papersize``
  - ``validate_svg_fonttype``
  - ``validate_toolbar``
  - ``validate_webagg_address``

- Some rcParam validation has become stricter:

  - :rc:`axes.axisbelow` no longer accepts strings starting with "line"
    (case-insensitive) as "line"; use "line" (case-sensitive) instead.
  - :rc:`text.latex.preamble` and ``pdf.preamble`` no longer accept non-string values.
  - All ``*.linestyle`` rcParams no longer accept ``offset = None``; set the
    offset to 0 instead.