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Sep 22, 2021
+34 −26
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Improve curve_error_band example. #21153

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60 changes: 34 additions & 26 deletions examples/lines_bars_and_markers/curve_error_band.py
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Original file line number Diff line number Diff line change
Expand Up @@ -10,7 +10,6 @@
# sphinx_gallery_thumbnail_number = 2

import numpy as np
from scipy.interpolate import splprep, splev

import matplotlib.pyplot as plt
from matplotlib.path import Path
Expand All @@ -22,8 +21,8 @@
x, y = r * np.cos(t), r * np.sin(t)

fig, ax = plt.subplots()
ax.plot(x, y)
plt.show()
ax.plot(x, y, "k")
ax.set(aspect=1)

#############################################################################
# An error band can be used to indicate the uncertainty of the curve.
Expand All @@ -39,34 +38,43 @@
# `~.Axes.fill_between` method (see also
# :doc:`/gallery/lines_bars_and_markers/fill_between_demo`).

# Error amplitudes depending on the curve parameter *t*
# (actual values are arbitrary and only for illustrative purposes):
err = 0.05 * np.sin(2 * t) ** 2 + 0.04 + 0.02 * np.cos(9 * t + 2)

# calculate normals via derivatives of splines
tck, u = splprep([x, y], s=0)
dx, dy = splev(u, tck, der=1)
l = np.hypot(dx, dy)
nx = dy / l
ny = -dx / l
def draw_error_band(ax, x, y, err, **kwargs):
# Calculate normals via centered finite differences (except the first point
# which uses a forward difference and the last point which uses a backward
# difference).
dx = np.concatenate([[x[1] - x[0]], x[2:] - x[:-2], [x[-1] - x[-2]]])
dy = np.concatenate([[y[1] - y[0]], y[2:] - y[:-2], [y[-1] - y[-2]]])
l = np.hypot(dx, dy)
nx = dy / l
ny = -dx / l

# end points of errors
xp = x + nx * err
yp = y + ny * err
xn = x - nx * err
yn = y - ny * err
# end points of errors
xp = x + nx * err
yp = y + ny * err
xn = x - nx * err
yn = y - ny * err

vertices = np.block([[xp, xn[::-1]],
[yp, yn[::-1]]]).T
codes = Path.LINETO * np.ones(len(vertices), dtype=Path.code_type)
codes[0] = codes[len(xp)] = Path.MOVETO
path = Path(vertices, codes)
vertices = np.block([[xp, xn[::-1]],
[yp, yn[::-1]]]).T
codes = np.full(len(vertices), Path.LINETO)
codes[0] = codes[len(xp)] = Path.MOVETO
path = Path(vertices, codes)
ax.add_patch(PathPatch(path, **kwargs))

patch = PathPatch(path, facecolor='C0', edgecolor='none', alpha=0.3)

fig, ax = plt.subplots()
ax.plot(x, y)
ax.add_patch(patch)
axs = (plt.figure(constrained_layout=True)
.subplots(1, 2, sharex=True, sharey=True))
errs = [
(axs[0], "constant error", 0.05),
(axs[1], "variable error", 0.05 * np.sin(2 * t) ** 2 + 0.04),
]
for i, (ax, title, err) in enumerate(errs):
ax.set(title=title, aspect=1, xticks=[], yticks=[])
ax.plot(x, y, "k")
draw_error_band(ax, x, y, err=err,
facecolor=f"C{i}", edgecolor="none", alpha=.3)

plt.show()

#############################################################################
Expand Down