Fix conversion of matplotlib contour lines

[robertoffmoura]

mpl_to_plotly doesn't render contour lines correctly. Matplotlib packs each contour level into a single path containing several disjoint subpaths, so the converted figure connects them with segments that shouldn't exist, and closed rings are left open because the CLOSEPOLY vertex is dropped on export:

• separate branches of the same level get joined by diagonals
• every closed loop misses its final segment

Fix: path collections with no face colors (line collections, which is what ax.contour produces) are now drawn as line traces instead of filled polygons:

• disjoint subpaths inside a path are separated with None, so plotly does not connect them
• subpaths ending in a Z code have their first vertex appended to close the ring
• multi-vertex path codes (C, S) consume the correct number of vertices while parsing
• degenerate paths (fewer than 2 vertices, e.g. the empty path some contour sets carry) are skipped
• x-values are converted to date strings on date axes, and per-path edge colors/linewidths are applied in order

Snippet to reproduce:

import matplotlib
matplotlib.use("Agg")
import matplotlib.pyplot as plt
import numpy as np
import plotly.tools as tls

x = np.linspace(-2, 2, 120)
X, Y = np.meshgrid(x, x)
Z = np.sin(3 * np.sqrt(X**2 + Y**2))

fig, ax = plt.subplots()
ax.contour(X, Y, Z, levels=[-0.75, -0.25, 0.25, 0.75], linewidths=2)
fig.savefig("contour_mpl.png")

p = tls.mpl_to_plotly(fig)
p.write_image("contour_plotly.png")

matplotlib	plotly before	plotly after

[camdecoster]

This looks good, but does it handle the case when a plot has a legend enabled?
Never mind, wrong PR window.

[camdecoster]

This looks good and seems like a fine solution. I added a few comments/questions about potential improvements.

Could you please add a changelog entry?

[camdecoster]

Could you please add a test for the None separators as described in the PR description?

[camdecoster]

This helper function looks like the same one from _draw_filled_path_collection. Could you move it up a level and reuse it in both locations?

[camdecoster]

Will this handle dash styles?

What about turning off legend display? I think that came up in another PR.

[camdecoster]

This function currently draws one trace per path. What do you think of updating it to group consecutive same-style lines together into one trace? That would cut down on the number of traces (and make a legend less noisy).