.. note::
    :class: sphx-glr-download-link-note

    Click :ref:`here <sphx_glr_download_gallery_lines_bars_and_markers_fill_between_demo.py>` to download the full example code
.. rst-class:: sphx-glr-example-title

.. _sphx_glr_gallery_lines_bars_and_markers_fill_between_demo.py:


==============================
Filling the area between lines
==============================

This example shows how to use `~.axes.Axes.fill_between` to color the area
between two lines.


.. code-block:: python


    import matplotlib.pyplot as plt
    import numpy as np







Basic usage
-----------
The parameters *y1* and *y2* can be a scalar, indicating a horizontal
boundary a  the given y-values. If only *y1* is given, *y2* defaults to 0.



.. code-block:: python


    x = np.arange(0.0, 2, 0.01)
    y1 = np.sin(2 * np.pi * x)
    y2 = 0.8 * np.sin(4 * np.pi * x)

    fig, (ax1, ax2, ax3) = plt.subplots(3, 1, sharex=True, figsize=(6, 6))

    ax1.fill_between(x, y1)
    ax1.set_title('fill between y1 and 0')

    ax2.fill_between(x, y1, 1)
    ax2.set_title('fill between y1 and 1')

    ax3.fill_between(x, y1, y2)
    ax3.set_title('fill between y1 and y2')
    ax3.set_xlabel('x')
    fig.tight_layout()




.. image:: /gallery/lines_bars_and_markers/images/sphx_glr_fill_between_demo_001.png
    :class: sphx-glr-single-img




Example: Confidence bands
-------------------------
A common application for `~.axes.Axes.fill_between` is the indication of
confidence bands.

`~.axes.Axes.fill_between` uses the colors of the color cycle as the fill
color. These may be a bit strong when applied to fill areas. It is
therefore often a good practice to lighten the color by making the area
semi-transparent using *alpha*.



.. code-block:: python


    # sphinx_gallery_thumbnail_number = 2

    N = 21
    x = np.linspace(0, 10, 11)
    y = [3.9, 4.4, 10.8, 10.3, 11.2, 13.1, 14.1,  9.9, 13.9, 15.1, 12.5]

    # fit a linear curve an estimate its y-values and their error.
    a, b = np.polyfit(x, y, deg=1)
    y_est = a * x + b
    y_err = x.std() * np.sqrt(1/len(x) +
                              (x - x.mean())**2 / np.sum((x - x.mean())**2))

    fig, ax = plt.subplots()
    ax.plot(x, y_est, '-')
    ax.fill_between(x, y_est - y_err, y_est + y_err, alpha=0.2)
    ax.plot(x, y, 'o', color='tab:brown')




.. image:: /gallery/lines_bars_and_markers/images/sphx_glr_fill_between_demo_002.png
    :class: sphx-glr-single-img




Selectively filling horizontal regions
--------------------------------------
The parameter *where* allows to specify the x-ranges to fill. It's a boolean
array with the same size as *x*.

Only x-ranges of contiguous *True* sequences are filled. As a result the
range between neighboring *True* and *False* values is never filled. This
often undesired when the data points should represent a contiguous quantity.
It is therefore recommended to set ``interpolate=True`` unless the
x-distance of the data points is fine enough so that the above effect is not
noticeable. Interpolation approximates the actual x position at which the
*where* condition will change and extends the filling up to there.



.. code-block:: python


    x = np.array([0, 1, 2, 3])
    y1 = np.array([0.8, 0.8, 0.2, 0.2])
    y2 = np.array([0, 0, 1, 1])

    fig, (ax1, ax2) = plt.subplots(2, 1, sharex=True)

    ax1.set_title('interpolation=False')
    ax1.plot(x, y1, 'o--')
    ax1.plot(x, y2, 'o--')
    ax1.fill_between(x, y1, y2, where=(y1 > y2), color='C0', alpha=0.3)
    ax1.fill_between(x, y1, y2, where=(y1 < y2), color='C1', alpha=0.3)

    ax2.set_title('interpolation=True')
    ax2.plot(x, y1, 'o--')
    ax2.plot(x, y2, 'o--')
    ax2.fill_between(x, y1, y2, where=(y1 > y2), color='C0', alpha=0.3,
                     interpolate=True)
    ax2.fill_between(x, y1, y2, where=(y1 <= y2), color='C1', alpha=0.3,
                     interpolate=True)
    fig.tight_layout()




.. image:: /gallery/lines_bars_and_markers/images/sphx_glr_fill_between_demo_003.png
    :class: sphx-glr-single-img




.. note::

   Similar gaps will occur if *y1* or *y2* are masked arrays. Since missing
   values cannot be approximated, *interpolate* has no effect in this case.
   The gaps around masked values can only be reduced by adding more data
   points close to the masked values.


Selectively marking horizontal regions across the whole Axes
------------------------------------------------------------
The same selection mechanism can be applied to fill the full vertical height
of the axes. To be independent of y-limits, we add a transform that
interprets the x-values in data coorindates and the y-values in axes
coordinates.

The following example marks the regions in which the y-data are above a
given threshold.



.. code-block:: python


    fig, ax = plt.subplots()
    x = np.arange(0, 4 * np.pi, 0.01)
    y = np.sin(x)
    ax.plot(x, y, color='black')

    threshold = 0.75
    ax.axhline(threshold, color='green', lw=2, alpha=0.7)
    ax.fill_between(x, 0, 1, where=y > threshold,
                    color='green', alpha=0.5, transform=ax.get_xaxis_transform())




.. image:: /gallery/lines_bars_and_markers/images/sphx_glr_fill_between_demo_004.png
    :class: sphx-glr-single-img




------------

References
""""""""""

The use of the following functions, methods and classes is shown
in this example:



.. code-block:: python


    import matplotlib
    matplotlib.axes.Axes.fill_between
    matplotlib.pyplot.fill_between
    matplotlib.axes.Axes.get_xaxis_transform







.. _sphx_glr_download_gallery_lines_bars_and_markers_fill_between_demo.py:


.. only :: html

 .. container:: sphx-glr-footer
    :class: sphx-glr-footer-example



  .. container:: sphx-glr-download

     :download:`Download Python source code: fill_between_demo.py <fill_between_demo.py>`



  .. container:: sphx-glr-download

     :download:`Download Jupyter notebook: fill_between_demo.ipynb <fill_between_demo.ipynb>`


.. only:: html

 .. rst-class:: sphx-glr-signature

    Keywords: matplotlib code example, codex, python plot, pyplot
    `Gallery generated by Sphinx-Gallery
    <https://sphinx-gallery.readthedocs.io>`_
