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Initializing WCSAxes with custom transforms
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In :ref:`initialization`, we saw how to make plots using
:class:`~astropy.wcs.WCS` objects. However, the
:class:`~astropy.visualization.wcsaxes.WCSAxes` class can also be initialized
with more general transformations that don't have to be represented by the
:class:`~astropy.wcs.WCS` class. Instead, you can initialize
:class:`~astropy.visualization.wcsaxes.WCSAxes` using a Matplotlib
:class:`~matplotlib.transforms.Transform` object and a dictionary
(``coord_meta``) that provides metadata on how to interpret the transformation.

The :class:`~matplotlib.transforms.Transform` should represent the conversion
from pixel to world coordinates, and should have ``input_dims=2`` and
``output_dims=2``. In addition, ``has_inverse`` should be set to `True` and
the ``inverted`` method should be implemented.

The ``coord_meta`` dictionary should include the following keys:

* ``name``: an iterable of two strings giving the names for each dimension
* ``type``: an iterable of two strings that should be either ``'longitude'``,
  ``'latitude'``, or ``'scalar'`` (for anything that isn't a longitude or latitude).
* ``wrap``: an iterable of two values which indicate for longitudes at which
  angle (in degrees) to wrap the coordinates. This should be `None` unless
  ``type`` is ``'longitude'``.
* ``unit``: an iterable of two :class:`~astropy.units.Unit` objects giving the
  units of the world coordinates returned by the
  :class:`~matplotlib.transforms.Transform`.
* ``format_unit``: an iterable of two :class:`~astropy.units.Unit` objects
  giving the units to use for the formatting of the labels. These can be set to
  `None` to default to the units given in ``unit``, but can be set for example
  if the :class:`~matplotlib.transforms.Transform` returns values in degrees
  and you want the labels to be formatted in hours.

The following example illustrates a custom projection using a transform and
``coord_meta``:

.. plot::
   :context: reset
   :include-source:
   :align: center

    from astropy import units as u
    import matplotlib.pyplot as plt
    from matplotlib.transforms import Affine2D
    from astropy.visualization.wcsaxes import WCSAxes

    # Set up an affine transformation
    transform = Affine2D()
    transform.scale(0.01)
    transform.translate(40, -30)
    transform.rotate(0.3)  # radians

    # Set up metadata dictionary
    coord_meta = {}
    coord_meta['name'] = 'lon', 'lat'
    coord_meta['type'] = 'longitude', 'latitude'
    coord_meta['wrap'] = 180, None
    coord_meta['unit'] = u.deg, u.deg
    coord_meta['format_unit'] = None, None

    fig = plt.figure()
    ax = WCSAxes(fig, [0.1, 0.1, 0.8, 0.8], aspect='equal',
                 transform=transform, coord_meta=coord_meta)
    fig.add_axes(ax)
    ax.set_xlim(-0.5, 499.5)
    ax.set_ylim(-0.5, 399.5)
    ax.grid()
    ax.coords['lon'].set_axislabel('Longitude')
    ax.coords['lat'].set_axislabel('Latitude')