Initializing WCSAxes with custom transforms

In Initializing axes with world coordinates, we saw how to make plots using WCS objects. However, the WCSAxes class can also be initialized with more general transformations that don’t have to be represented by the WCS class. Instead, you can initialize WCSAxes using a Matplotlib Transform object and a dictionary (coord_meta) that provides metadata on how to interpret the transformation.

The 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 Unit objects giving the units of the world coordinates returned by the Transform.
• format_unit: an iterable of two 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 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:

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')

()