From 9c3a78e09ea872be5559c833ee66fdd81cbdf84d Mon Sep 17 00:00:00 2001
From: kolibril13 <44469195+kolibril13@users.noreply.github.com>
Date: Tue, 17 Dec 2019 00:59:41 +0100
Subject: [PATCH] Backport PR #15914: Example for sigmoid function with
 horizontal lines

---
 .flake8                    |  1 +
 examples/pyplots/axline.py | 41 ++++++++++++++++++++++++++++++++++++++
 2 files changed, 42 insertions(+)
 create mode 100644 examples/pyplots/axline.py

diff --git a/.flake8 b/.flake8
index b09f54149d93..4c1f89a6a946 100644
--- a/.flake8
+++ b/.flake8
@@ -165,6 +165,7 @@ per-file-ignores =
     examples/pyplots/annotation_basic.py: E402
     examples/pyplots/annotation_polar.py: E231, E402
     examples/pyplots/auto_subplots_adjust.py: E231, E302, E402
+    examples/pyplots/axline.py: E402
     examples/pyplots/boxplot_demo_pyplot.py: E231, E402
     examples/pyplots/compound_path_demo.py: E231
     examples/pyplots/dollar_ticks.py: E402
diff --git a/examples/pyplots/axline.py b/examples/pyplots/axline.py
new file mode 100644
index 000000000000..73eed0752c7d
--- /dev/null
+++ b/examples/pyplots/axline.py
@@ -0,0 +1,41 @@
+"""
+======================================
+Infinite horizontal and vertical lines
+======================================
+
+`~.axes.Axes.axvline` and `~.axes.Axes.axhline` draw infinite vertical /
+horizontal lines, at given *x* / *y* positions. They are usually used to mark
+special data values, e.g. in this example the center and limit values of the
+sigmoid function.
+"""
+import numpy as np
+import matplotlib.pyplot as plt
+
+t = np.linspace(-10, 10, 100)
+sig = 1 / (1 + np.exp(-t))
+
+plt.axhline(y=0, color="black", linestyle="--")
+plt.axhline(y=0.5, color="black", linestyle=":")
+plt.axhline(y=1.0, color="black", linestyle="--")
+plt.axvline(color="grey")
+plt.plot(t, sig, linewidth=2, label=r"$\sigma(t) = \frac{1}{1 + e^{-t}}$")
+plt.xlim(-10, 10)
+plt.xlabel("t")
+plt.legend(fontsize=14)
+plt.show()
+
+#############################################################################
+#
+# ------------
+#
+# References
+# """"""""""
+#
+# The use of the following functions, methods, classes and modules is shown
+# in this example:
+
+import matplotlib
+matplotlib.pyplot.axhline
+matplotlib.pyplot.axvline
+matplotlib.axes.Axes.axhline
+matplotlib.axes.Axes.axvline