Export tick label colors to the plotly tickfont

[robertoffmoura]

Tick label colors are dropped by the converter. The mplexporter does not export the tick label color, so the plotly tickfont gets a size with no color and tick labels always render in plotly's default dark grey, which is nearly invisible on dark figures (e.g. plt.style.use("dark_background")).

Fix: the mplexporter axis properties now export the tick label color, and prep_xy_axis passes it through to the plotly tickfont.

Snippet to reproduce:

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

plt.style.use("dark_background")

fig, ax = plt.subplots()
ax.plot(np.sin(np.linspace(0, 6, 100)))
fig.savefig("ticks_mpl.png")

p = tls.mpl_to_plotly(fig)   # tickfont.color was None before the fix
p.write_image("ticks_plotly.png")

matplotlib	plotly before	plotly after

[camdecoster]

Transparent colors strike again! Could you please address the comments and add a changelog entry? Thanks for all of your work in this area.

[camdecoster]

export_color can produce "none" for transparent labels. As is, this could crash the conversion.

[camdecoster]

You could wrap this (and below) in _export_color to address the "none" color issue.