.. only:: html
.. note::
:class: sphx-glr-download-link-note
Click :ref:`here ` to download the full example code
.. rst-class:: sphx-glr-example-title
.. _sphx_glr_gallery_lines_bars_and_markers_curve_error_band.py:
=====================
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`.
.. code-block:: default
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()
.. image:: /gallery/lines_bars_and_markers/images/sphx_glr_curve_error_band_001.png
:alt: curve error band
:class: sphx-glr-single-img
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`).
.. code-block:: default
# 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()
.. image:: /gallery/lines_bars_and_markers/images/sphx_glr_curve_error_band_002.png
:alt: curve error band
:class: sphx-glr-single-img
------------
References
""""""""""
The use of the following functions, methods and classes is shown
in this example:
.. code-block:: default
import matplotlib
matplotlib.patches.PathPatch
matplotlib.path.Path
.. _sphx_glr_download_gallery_lines_bars_and_markers_curve_error_band.py:
.. only :: html
.. container:: sphx-glr-footer
:class: sphx-glr-footer-example
.. container:: sphx-glr-download sphx-glr-download-python
:download:`Download Python source code: curve_error_band.py `
.. container:: sphx-glr-download sphx-glr-download-jupyter
:download:`Download Jupyter notebook: curve_error_band.ipynb `
.. only:: html
.. rst-class:: sphx-glr-signature
Keywords: matplotlib code example, codex, python plot, pyplot
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