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Cleanup custom_scale example. #16240

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Jan 16, 2020
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Cleanup custom_scale example. #16240

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65 changes: 26 additions & 39 deletions examples/scales/custom_scale.py
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Original file line number Diff line number Diff line change
Expand Up @@ -7,28 +7,22 @@
Mercator Projection.

Unless you are making special use of the `~.Transform` class, you probably
don't need to use this verbose method, and instead can use
`~.matplotlib.scale.FuncScale` and the ``'function'`` option of
`~.matplotlib.axes.Axes.set_xscale` and `~.matplotlib.axes.Axes.set_yscale`.
don't need to use this verbose method, and instead can use `~.scale.FuncScale`
and the ``'function'`` option of `~.Axes.set_xscale` and `~.Axes.set_yscale`.
See the last example in :doc:`/gallery/scales/scales`.
"""

import numpy as np
from numpy import ma
from matplotlib import scale as mscale
from matplotlib import transforms as mtransforms
from matplotlib.ticker import Formatter, FixedLocator
from matplotlib import rcParams


# BUG: this example fails with any other setting of axisbelow
rcParams['axes.axisbelow'] = False
from matplotlib.ticker import FixedLocator, FuncFormatter


class MercatorLatitudeScale(mscale.ScaleBase):
"""
Scales data in range -pi/2 to pi/2 (-90 to 90 degrees) using
the system used to scale latitudes in a Mercator projection.
the system used to scale latitudes in a Mercator__ projection.

The scale function:
ln(tan(y) + sec(y))
Expand All @@ -40,8 +34,7 @@ class MercatorLatitudeScale(mscale.ScaleBase):
there is user-defined threshold, above and below which nothing
will be plotted. This defaults to +/- 85 degrees.

source:
http://en.wikipedia.org/wiki/Mercator_projection
__ http://en.wikipedia.org/wiki/Mercator_projection
"""

# The scale class must have a member ``name`` that defines the string used
Expand Down Expand Up @@ -77,21 +70,16 @@ def set_default_locators_and_formatters(self, axis):
scale. This is only required if the scale requires custom
locators and formatters. Writing custom locators and
formatters is rather outside the scope of this example, but
there are many helpful examples in ``ticker.py``.
there are many helpful examples in :mod:`.ticker`.

In our case, the Mercator example uses a fixed locator from
-90 to 90 degrees and a custom formatter class to put convert
the radians to degrees and put a degree symbol after the
value::
In our case, the Mercator example uses a fixed locator from -90 to 90
degrees and a custom formatter to convert the radians to degrees and
put a degree symbol after the value.
"""
class DegreeFormatter(Formatter):
def __call__(self, x, pos=None):
return "%d\N{DEGREE SIGN}" % np.degrees(x)

axis.set_major_locator(FixedLocator(
np.radians(np.arange(-90, 90, 10))))
axis.set_major_formatter(DegreeFormatter())
axis.set_minor_formatter(DegreeFormatter())
fmt = FuncFormatter(
lambda x, pos=None: f"{np.degrees(x):.0f}\N{DEGREE SIGN}")
axis.set(major_locator=FixedLocator(np.radians(range(-90, 90, 10))),
major_formatter=fmt, minor_formatter=fmt)

def limit_range_for_scale(self, vmin, vmax, minpos):
"""
Expand Down Expand Up @@ -122,25 +110,24 @@ def __init__(self, thresh):

def transform_non_affine(self, a):
"""
This transform takes an Nx1 ``numpy`` array and returns a
transformed copy. Since the range of the Mercator scale
is limited by the user-specified threshold, the input
array must be masked to contain only valid values.
``matplotlib`` will handle masked arrays and remove the
out-of-range data from the plot. Importantly, the
``transform`` method *must* return an array that is the
same shape as the input array, since these values need to
remain synchronized with values in the other dimension.
This transform takes a numpy array and returns a transformed copy.
Since the range of the Mercator scale is limited by the
user-specified threshold, the input array must be masked to
contain only valid values. Matplotlib will handle masked arrays
and remove the out-of-range data from the plot. However, the
returned array *must* have the same shape as the input array, since
these values need to remain synchronized with values in the other
dimension.
"""
masked = ma.masked_where((a < -self.thresh) | (a > self.thresh), a)
if masked.mask.any():
return ma.log(np.abs(ma.tan(masked) + 1.0 / ma.cos(masked)))
return ma.log(np.abs(ma.tan(masked) + 1 / ma.cos(masked)))
else:
return np.log(np.abs(np.tan(a) + 1.0 / np.cos(a)))
return np.log(np.abs(np.tan(a) + 1 / np.cos(a)))

def inverted(self):
"""
Override this method so matplotlib knows how to get the
Override this method so Matplotlib knows how to get the
inverse transform for this transform.
"""
return MercatorLatitudeScale.InvertedMercatorLatitudeTransform(
Expand All @@ -161,7 +148,7 @@ def inverted(self):


# Now that the Scale class has been defined, it must be registered so
# that ``matplotlib`` can find it.
# that Matplotlib can find it.
mscale.register_scale(MercatorLatitudeScale)


Expand All @@ -176,7 +163,7 @@ def inverted(self):

plt.xlabel('Longitude')
plt.ylabel('Latitude')
plt.title('Mercator: Projection of the Oppressor')
plt.title('Mercator projection')
plt.grid(True)

plt.show()