>>> """
==========
Streamplot
==========

A stream plot, or streamline plot, is used to display 2D vector fields. This
example shows a few features of the `~.axes.Axes.streamplot` function:

* Varying the color along a streamline.
* Varying the density of streamlines.
* Varying the line width along a streamline.
* Controlling the starting points of streamlines.
* Streamlines skipping masked regions and NaN values.
"""
... import numpy as np
... import matplotlib.pyplot as plt
... import matplotlib.gridspec as gridspec
... 
... w = 3
... Y, X = np.mgrid[-w:w:100j, -w:w:100j]
... U = -1 - X**2 + Y
... V = 1 + X - Y**2
... speed = np.sqrt(U**2 + V**2)
... 
... fig = plt.figure(figsize=(7, 9))
... gs = gridspec.GridSpec(nrows=3, ncols=2, height_ratios=[1, 1, 2])
... 
... #  Varying density along a streamline
... ax0 = fig.add_subplot(gs[0, 0])
... ax0.streamplot(X, Y, U, V, density=[0.5, 1])
... ax0.set_title('Varying Density')
... 
... # Varying color along a streamline
... ax1 = fig.add_subplot(gs[0, 1])
... strm = ax1.streamplot(X, Y, U, V, color=U, linewidth=2, cmap='autumn')
... fig.colorbar(strm.lines)
... ax1.set_title('Varying Color')
... 
... #  Varying line width along a streamline
... ax2 = fig.add_subplot(gs[1, 0])
... lw = 5*speed / speed.max()
... ax2.streamplot(X, Y, U, V, density=0.6, color='k', linewidth=lw)
... ax2.set_title('Varying Line Width')
... 
... # Controlling the starting points of the streamlines
... seed_points = np.array([[-2, -1, 0, 1, 2, -1], [-2, -1,  0, 1, 2, 2]])
... 
... ax3 = fig.add_subplot(gs[1, 1])
... strm = ax3.streamplot(X, Y, U, V, color=U, linewidth=2,
...                       cmap='autumn', start_points=seed_points.T)
... fig.colorbar(strm.lines)
... ax3.set_title('Controlling Starting Points')
... 
... # Displaying the starting points with blue symbols.
... ax3.plot(seed_points[0], seed_points[1], 'bo')
... ax3.set(xlim=(-w, w), ylim=(-w, w))
... 
... # Create a mask
... mask = np.zeros(U.shape, dtype=bool)
... mask[40:60, 40:60] = True
... U[:20, :20] = np.nan
... U = np.ma.array(U, mask=mask)
... 
... ax4 = fig.add_subplot(gs[2:, :])
... ax4.streamplot(X, Y, U, V, color='r')
... ax4.set_title('Streamplot with Masking')
... 
... ax4.imshow(~mask, extent=(-w, w, -w, w), alpha=0.5, cmap='gray', aspect='auto')
... ax4.set_aspect('equal')
... 
... plt.tight_layout()
... plt.show()
... #############################################################################
... #
... # .. admonition:: References
... #
... #    The use of the following functions, methods, classes and modules is shown
... #    in this example:
... #
... #    - `matplotlib.axes.Axes.streamplot` / `matplotlib.pyplot.streamplot`
... #    - `matplotlib.gridspec.GridSpec`
...