From 05b786c0b69b250e5d1bf85708cf7ca23106080c Mon Sep 17 00:00:00 2001
From: Tim Hoffmann <2836374+timhoffm@users.noreply.github.com>
Date: Tue, 19 May 2020 00:22:39 +0200
Subject: [PATCH] Add example for drawing an error band around a curve

---
 .flake8                                       |  1 +
 .../curve_error_band.py                       | 84 +++++++++++++++++++
 2 files changed, 85 insertions(+)
 create mode 100644 examples/lines_bars_and_markers/curve_error_band.py

diff --git a/.flake8 b/.flake8
index be20a443016d..2949473c69e9 100644
--- a/.flake8
+++ b/.flake8
@@ -158,6 +158,7 @@ per-file-ignores =
     examples/images_contours_and_fields/tripcolor_demo.py: E201, E402
     examples/images_contours_and_fields/triplot_demo.py: E201, E402
     examples/images_contours_and_fields/watermark_image.py: E402
+    examples/lines_bars_and_markers/curve_error_band.py: E402
     examples/lines_bars_and_markers/errorbar_limits_simple.py: E402
     examples/lines_bars_and_markers/fill.py: E402
     examples/lines_bars_and_markers/fill_between_demo.py: E402
diff --git a/examples/lines_bars_and_markers/curve_error_band.py b/examples/lines_bars_and_markers/curve_error_band.py
new file mode 100644
index 000000000000..3d1675570b24
--- /dev/null
+++ b/examples/lines_bars_and_markers/curve_error_band.py
@@ -0,0 +1,84 @@
+"""
+=====================
+Curve with error band
+=====================
+
+This example illustrates how to draw an error band around a parametrized curve.
+
+A parametrized curve x(t), y(t) can directly be drawn using `~.Axes.plot`.
+"""
+# 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
+from matplotlib.patches import PathPatch
+
+N = 400
+t = np.linspace(0, 2 * np.pi, N)
+r = 0.5 + np.cos(t)
+x, y = r * np.cos(t), r * np.sin(t)
+
+fig, ax = plt.subplots()
+ax.plot(x, y)
+plt.show()
+
+#############################################################################
+# An error band can be used to indicate the uncertainty of the curve.
+# In this example we assume that the error can be given as a scalar *err*
+# that describes the uncertainty perpendicular to the curve in every point.
+#
+# We visualize this error as a colored band around the path using a
+# `.PathPatch`. The patch is created from two path segments *(xp, yp)*, and
+# *(xn, yn)* that are shifted by +/- *err* perpendicular to the curve *(x, y)*.
+#
+# Note: This method of using a `.PathPatch` is suited to arbitrary curves in
+# 2D. If you just have a standard y-vs.-x plot, you can use the simpler
+# `~.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
+
+# 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)
+
+patch = PathPatch(path, facecolor='C0', edgecolor='none', alpha=0.3)
+
+fig, ax = plt.subplots()
+ax.plot(x, y)
+ax.add_patch(patch)
+plt.show()
+
+#############################################################################
+#
+# ------------
+#
+# References
+# """"""""""
+#
+# The use of the following functions, methods and classes is shown
+# in this example:
+
+import matplotlib
+matplotlib.patches.PathPatch
+matplotlib.path.Path