Fix boxplot conversion by mapping 'none' colors to transparent rgba

[robertoffmoura]

mpl_to_plotly crashes when converting boxplots:

ValueError: Invalid value of type 'builtins.str' received for the 'color' property of scatter.marker
    Received value: 'none'

Boxplot outlier markers use facecolor="none" (fully transparent) in matplotlib. The converter passes that color verbatim into the plotly trace, and plotly.py's validators reject "none" for color property.

Fix: transparent matplotlib colors are mapped to rgba(0,0,0,0), which plotly accepts.

Snippet to reproduce:

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

fig, ax = plt.subplots()
ax.boxplot(np.random.randn(100, 4))
fig.savefig("boxplot_mpl.png")

p = tls.mpl_to_plotly(fig)   # raised ValueError before the fix
p.write_image("boxplot_plotly.png")

[camdecoster]

Thanks for another update! I left some suggestions that I think would simplify/improve the code.

On another note, what do you think of updating our copy of mplexporter? It wouldn't be simple, but it might be worth the effort to take advantage of the updates that have been made in the last 12 years.

[camdecoster]

This will still throw if the line color has an alpha value, which can happen with mplexporter (because the returned color is rgba and merge_color_and_opacity calls hex_to_rgb). This is a preexisting bug, but you might as well address it since you're in this area.

[camdecoster]

What do you think of combining merge_color_and_opacity and _export_color? They seem similar enough and you could deal with converting "none" internally rather than having to add a special case. Additionally, merge_color_and_opacity only gets called once, so we could remove that and replace it with the combined function.

[camdecoster]

It looks like you could run into the same issue with "none" colors for text. That might be rare, but it could still happen. You can see this on L735 where color=props["style"]["color"] gets passed in without being run through _export_color. I'll let you decide if you make an update in this PR or a future one.

[camdecoster]

Could you also please add a changelog entry?