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# Filling the area between lines

This example shows how to use `fill_between` to color between lines based on user-defined logic.

    import matplotlib.pyplot as plt
    import numpy as np

    x = np.arange(0.0, 2, 0.01)
    y1 = np.sin(2*np.pi*x)
    y2 = 1.2*np.sin(4*np.pi*x)

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

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

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

    ax3.fill_between(x, y1, y2)
    ax3.set_ylabel('between y1 and y2')
    ax3.set_xlabel('x')

    # now fill between y1 and y2 where a logical condition is met.  Note
    # this is different than calling
    #   fill_between(x[where], y1[where],y2[where]
    # because of edge effects over multiple contiguous regions.
    fig, (ax, ax1) = plt.subplots(2, 1, sharex=True)
    ax.plot(x, y1, x, y2, color='black')
    ax.fill_between(x, y1, y2, where=y2 >= y1, facecolor='green', interpolate=True)
    ax.fill_between(x, y1, y2, where=y2 <= y1, facecolor='red', interpolate=True)
    ax.set_title('fill between where')

    # Test support for masked arrays.
    y2 = np.ma.masked_greater(y2, 1.0)
    ax1.plot(x, y1, x, y2, color='black')
    ax1.fill_between(x, y1, y2, where=y2 >= y1, facecolor='green', interpolate=True)
    ax1.fill_between(x, y1, y2, where=y2 <= y1, facecolor='red', interpolate=True)
    ax1.set_title('Now regions with y2>1 are masked')

    # This example illustrates a problem; because of the data
    # gridding, there are undesired unfilled triangles at the crossover
    # points.  A brute-force solution would be to interpolate all
    # arrays to a very fine grid before plotting.

    # show how to use transforms to create axes spans where a certain condition is satisfied
    fig, ax = plt.subplots()
    y = np.sin(4*np.pi*x)
    ax.plot(x, y, color='black')

    # use the data coordinates for the x-axis and the axes coordinates for the y-axis
    import matplotlib.transforms as mtransforms
    trans = mtransforms.blended_transform_factory(ax.transData, ax.transAxes)
    theta = 0.9
    ax.axhline(theta, color='green', lw=2, alpha=0.5)
    ax.axhline(-theta, color='red', lw=2, alpha=0.5)
    ax.fill_between(x, 0, 1, where=y > theta, facecolor='green', alpha=0.5, transform=trans)
    ax.fill_between(x, 0, 1, where=y < -theta, facecolor='red', alpha=0.5, transform=trans)

    plt.show()

Total running time of the script: ( 0 minutes 0.170 seconds)

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