How to plot a gradient color line in matplotlib?

Question

To state it in a general form, I'm looking for a way to join several points with a gradient color line using matplotlib, and I'm not finding it anywhere. To be more specific, I'm plotting a 2D random walk with a one color line. But, as the points have a relevant sequence, I would like to look at the plot and see where the data has moved. A gradient colored line would do the trick. Or a line with gradually changing transparency.

I'm just trying to improve the vizualization of my data. Check out this beautiful image produced by the ggplot2 package of R. I'm looking for the same in matplotlib. Thanks.

Answer 1

Note that if you have many points, calling plt.plot for each line segment can be quite slow. It's more efficient to use a LineCollection object.

Using the colorline recipe you could do the following:

import matplotlib.pyplot as plt
import numpy as np
import matplotlib.collections as mcoll
import matplotlib.path as mpath

def colorline(
    x, y, z=None, cmap=plt.get_cmap('copper'), norm=plt.Normalize(0.0, 1.0),
        linewidth=3, alpha=1.0):
    """
    http://nbviewer.ipython.org/github/dpsanders/matplotlib-examples/blob/master/colorline.ipynb
    http://matplotlib.org/examples/pylab_examples/multicolored_line.html
    Plot a colored line with coordinates x and y
    Optionally specify colors in the array z
    Optionally specify a colormap, a norm function and a line width
    """

    # Default colors equally spaced on [0,1]:
    if z is None:
        z = np.linspace(0.0, 1.0, len(x))

    # Special case if a single number:
    if not hasattr(z, "__iter__"):  # to check for numerical input -- this is a hack
        z = np.array([z])

    z = np.asarray(z)

    segments = make_segments(x, y)
    lc = mcoll.LineCollection(segments, array=z, cmap=cmap, norm=norm,
                              linewidth=linewidth, alpha=alpha)

    ax = plt.gca()
    ax.add_collection(lc)

    return lc


def make_segments(x, y):
    """
    Create list of line segments from x and y coordinates, in the correct format
    for LineCollection: an array of the form numlines x (points per line) x 2 (x
    and y) array
    """

    points = np.array([x, y]).T.reshape(-1, 1, 2)
    segments = np.concatenate([points[:-1], points[1:]], axis=1)
    return segments

N = 10
np.random.seed(101)
x = np.random.rand(N)
y = np.random.rand(N)
fig, ax = plt.subplots()

path = mpath.Path(np.column_stack([x, y]))
verts = path.interpolated(steps=3).vertices
x, y = verts[:, 0], verts[:, 1]
z = np.linspace(0, 1, len(x))
colorline(x, y, z, cmap=plt.get_cmap('jet'), linewidth=2)

plt.show()