Question

I’m making some contour plots in matplotlib and the length of the dashes are too long. The dotted line also doesn’t look good. I’d like to manually set the length of the dash. I can set the exact dash length when I’m making a simple plot using plt.plot(), however I cannot figure out how to do the same thing with a contour plot.

I think that the following code should work, but I get the error:

File "/Library/Python/2.7/site-packages/matplotlib-1.2.x-py2.7-macosx-10.8-intel.egg/matplotlib/backends/backend_macosx.py", line 80, in draw_path_collection
    offset_position)
TypeError: failed to obtain the offset and dashes from the linestyle

Here is a sample of what I’m trying to do, adapted from the MPL examples:

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


delta = 0.025
x = np.arange(-3.0, 3.0, delta)
y = np.arange(-2.0, 2.0, delta)
X, Y = np.meshgrid(x, y)
Z1 = mlab.bivariate_normal(X, Y, 1.0, 1.0, 0.0, 0.0)
Z2 = mlab.bivariate_normal(X, Y, 1.5, 0.5, 1, 1)
# difference of Gaussians
Z = 10.0 * (Z2 - Z1)

plt.figure()

CS = plt.contour(X, Y, Z, 6, colors='k',linestyles='dashed')

for c in CS.collections:
    c.set_dashes([2,2])

plt.show()

Thanks!

Answer 1

Almost.

It’s:

for c in CS.collections:
    c.set_dashes([(0, (2.0, 2.0))])

If you had put a print c.get_dashes() there, you would have found out (it’s what I did).

Perhaps the definition of the line style has changed a bit, and you were working from an older example.

The collections documentation has this to say:

• set_dashes(ls)

  alias for set_linestyle

• set_linestyle(ls)

  Set the linestyle(s) for the collection.

  ACCEPTS: [‘solid’ | ‘dashed’, ‘dashdot’, ‘dotted’ | (offset, on-off-dash-seq) ]

So in [(0, (2.0, 2.0))], 0 is the offset, and then the tuple is the on-off repeating pattern.