Dotted grid lines in v2.0.0b1

[rayosborn]

After installing v2.0.0b1 into my conda installation (Mac OS X 10.11.5, Python 2.7.11, Anaconda 2.3.0), I noticed a subtle problem with dotted lines in grids. I used the following code to generate the two attached images in v1.5.1 and v2.0.0b1, respectively:

import numpy as np
import matplotlib.pyplot as plt

x = np.array([1, 1, 1, 2, 2, 2, 3, 3, 3])
y = np.array([1, 2, 3, 1, 2, 3, 1, 2, 3])
a = np.array([1, 2, 3, 4, 5, 6, 7, 8, 9])
grid = a.reshape(3, 3)

plt.imshow(grid, extent=(x.min(), x.max(), y.min(), y.max()),interpolation='bicubic')
plt.grid(linestyle='dotted')
plt.show()

If you blow up the new image, alternate dots are linked by a faint gray line, whereas the old dots are clearly separated. Overlaid on some plots, this can make the gridlines look more like dashed lines.

[jenshnielsen]

This is caused by the changes in #5926 where we now changed the default pattern and made them scale with line-width. When the dash/dotted pattern is to small the anti aliasing causes the dots to overlap.

I still think that scaling with line width is a good idea but perhaps we should make the minimum patterns somewhat larger.

[tacaswell]

The solution is to change the default dot spacing to [1.1, 1.1], PR (plus justification plots) coming.

[efiring]

And change the min value to 2.

[tacaswell]

Under #6547 this code gives

[afvincent]

@tacaswell : isn't the patterns a bit flat compared to the aspect ratio of the dotted line style in the picture that you just posted on #6547? If I paste the orange dotted line with lw=1.555… from your example in #6547 over the example showing the new grid lines, here is what I get:

The orange dots (with ~ default lw) look much more like dots to me than the thiner black ones. I guess this is the result of combining a very thin linewidth value for the grid lines and the new scale floor of lw=2. The black ones may be a bit surprising when one asked for a dotted grid.

But anyway, for what it's worth, IMO the new grid style is better than before.

(NB: comparison picture done with Gimp, zoom is 400% and no interpolation was used.)

[tacaswell]

Closing as resolved

@rayosborn Can you check the 2.0.0b2 to verify that it actually fixes this for you?

[rayosborn]

It looks more consistent, and is certainly distinct from dashed. I'm not sure it really looks dotted to me - more like short-dashed. I think to look truly dotted, you probably have to have larger gaps between each dash, but I guess that is a matter of taste. I can live with this.

[fkbreitl]

After testing mpl 2.0.0 I realize the same problem of dotted grid lines not looking dotted as in earlier versions, but dashed.
Could you please realize a dotted appearance of grid lines and reopen this ticket.
The current style is not satisfying and a step back.