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

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

.. _sphx_glr_gallery_images_contours_and_fields_irregulardatagrid.py:


=======================================
Contour plot of irregularly spaced data
=======================================

Comparison of a contour plot of irregularly spaced data interpolated
on a regular grid versus a tricontour plot for an unstructured triangular grid.

Since `~.axes.Axes.contour` and `~.axes.Axes.contourf` expect the data to live
on a regular grid, plotting a contour plot of irregularly spaced data requires
different methods. The two options are:

* Interpolate the data to a regular grid first. This can be done with on-board
  means, e.g. via `~.tri.LinearTriInterpolator` or using external functionality
  e.g. via `scipy.interpolate.griddata`. Then plot the interpolated data with
  the usual `~.axes.Axes.contour`.
* Directly use `~.axes.Axes.tricontour` or `~.axes.Axes.tricontourf` which will
  perform a triangulation internally.

This example shows both methods in action.



.. code-block:: python


    import matplotlib.pyplot as plt
    import matplotlib.tri as tri
    import numpy as np

    np.random.seed(19680801)
    npts = 200
    ngridx = 100
    ngridy = 200
    x = np.random.uniform(-2, 2, npts)
    y = np.random.uniform(-2, 2, npts)
    z = x * np.exp(-x**2 - y**2)

    fig, (ax1, ax2) = plt.subplots(nrows=2)

    # -----------------------
    # Interpolation on a grid
    # -----------------------
    # A contour plot of irregularly spaced data coordinates
    # via interpolation on a grid.

    # Create grid values first.
    xi = np.linspace(-2.1, 2.1, ngridx)
    yi = np.linspace(-2.1, 2.1, ngridy)

    # Perform linear interpolation of the data (x,y)
    # on a grid defined by (xi,yi)
    triang = tri.Triangulation(x, y)
    interpolator = tri.LinearTriInterpolator(triang, z)
    Xi, Yi = np.meshgrid(xi, yi)
    zi = interpolator(Xi, Yi)

    # Note that scipy.interpolate provides means to interpolate data on a grid
    # as well. The following would be an alternative to the four lines above:
    #from scipy.interpolate import griddata
    #zi = griddata((x, y), z, (xi[None,:], yi[:,None]), method='linear')


    ax1.contour(xi, yi, zi, levels=14, linewidths=0.5, colors='k')
    cntr1 = ax1.contourf(xi, yi, zi, levels=14, cmap="RdBu_r")

    fig.colorbar(cntr1, ax=ax1)
    ax1.plot(x, y, 'ko', ms=3)
    ax1.axis((-2, 2, -2, 2))
    ax1.set_title('grid and contour (%d points, %d grid points)' %
                  (npts, ngridx * ngridy))


    # ----------
    # Tricontour
    # ----------
    # Directly supply the unordered, irregularly spaced coordinates
    # to tricontour.

    ax2.tricontour(x, y, z, levels=14, linewidths=0.5, colors='k')
    cntr2 = ax2.tricontourf(x, y, z, levels=14, cmap="RdBu_r")

    fig.colorbar(cntr2, ax=ax2)
    ax2.plot(x, y, 'ko', ms=3)
    ax2.axis((-2, 2, -2, 2))
    ax2.set_title('tricontour (%d points)' % npts)

    plt.subplots_adjust(hspace=0.5)
    plt.show()




.. image:: /gallery/images_contours_and_fields/images/sphx_glr_irregulardatagrid_001.png
    :class: sphx-glr-single-img




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

References
""""""""""

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



.. code-block:: python


    import matplotlib
    matplotlib.axes.Axes.contour
    matplotlib.pyplot.contour
    matplotlib.axes.Axes.contourf
    matplotlib.pyplot.contourf
    matplotlib.axes.Axes.tricontour
    matplotlib.pyplot.tricontour
    matplotlib.axes.Axes.tricontourf
    matplotlib.pyplot.tricontourf







.. _sphx_glr_download_gallery_images_contours_and_fields_irregulardatagrid.py:


.. only :: html

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



  .. container:: sphx-glr-download

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



  .. container:: sphx-glr-download

     :download:`Download Jupyter notebook: irregulardatagrid.ipynb <irregulardatagrid.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>`_
