Preserve matplotlib axis line colors when converting from matplotlib

[robertoffmoura]

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[emilykl]

@robertoffmoura Thanks for the contribution!

I'm happy to merge this but just to make sure I have the full context, can you explain why this is the correct behavior? Are the axes lines black by default in matplotlib?

[robertoffmoura]

Hi @emilykl,
Yes, the axes lines are black by default in matplotlib.

[robertoffmoura]

Hi @emilykl,

I updated the PR so that the plotly picks the axis color from the matplotlib figure. Before the change, the plotly plot axis was white even if the matplotlib plot axis was black.

Here's a snippet. Before the change it would print None None for both figures.

import matplotlib.pyplot as plt
import plotly.tools as tls

# default matplotlib axis colors -> black
fig1, ax1 = plt.subplots()
ax1.plot([0, 1], [0, 1])
p1 = tls.mpl_to_plotly(fig1)
print(p1.layout.xaxis.linecolor, p1.layout.yaxis.linecolor)  # #000000 #000000
fig1.savefig("matplotlib.png")
p1.write_image("plotly.png")

# custom spine colors -> faithfully preserved
fig2, ax2 = plt.subplots()
ax2.spines["bottom"].set_color("red")
ax2.spines["left"].set_color("green")
ax2.plot([0, 1], [0, 1])
p2 = tls.mpl_to_plotly(fig2)
print(p2.layout.xaxis.linecolor, p2.layout.yaxis.linecolor)  # #FF0000 #008000

Here's a screenshot of the matplotlib figure:

And here's a screenshot of the converted plotly figure before the change (incorrect white axis line)

After the change, it correctly displays the black axis line

[robertclaus]

Assigning @camdecoster since he now has context across the other active fix PRs in this area.