Use matplotlib background colors when converting from matplotlib

[robertoffmoura]

Code PR

• I have read through the contributing notes and understand the structure of the package. In particular, if my PR modifies code of plotly.graph_objects, my modifications concern the code generator and not the generated files.
• I have added tests or modified existing tests.
• For a new feature, I have added documentation examples (please see the doc checklist as well).
• I have added a CHANGELOG entry if changing anything substantial.
• For a new feature or a change in behavior, I have updated the relevant docstrings in the code.

[emilykl]

Hi @robertoffmoura , the default for layout.plot_bgcolor is already white, do you have an example of where the current default is not working?

[robertclaus]

@robertoffmoura given the age and last comment in this PR - should it be closed?

[robertoffmoura]

Hi @robertclaus,

I updated the PR to set plot_bgcolor and paper_bgcolor based on fig.patch.get_facecolor() and props["axesbg"].

The below snippet prints None None for both figures before the change, and prints the expected color after the change.

Snippet:

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

# default matplotlib backgrounds -> white
fig1, ax1 = plt.subplots()
ax1.plot([0, 1], [0, 1])
p1 = tls.mpl_to_plotly(fig1)
print(p1.layout.plot_bgcolor, p1.layout.paper_bgcolor)  # #FFFFFF #FFFFFF
fig1.savefig("matplotlib.png")
p1.write_image("plotly.png")

# custom backgrounds -> faithfully preserved
fig2, ax2 = plt.subplots()
fig2.patch.set_facecolor("lightyellow")
ax2.set_facecolor("lightgray")
ax2.plot([0, 1], [0, 1])
p2 = tls.mpl_to_plotly(fig2)
print(p2.layout.plot_bgcolor, p2.layout.paper_bgcolor)  # #D3D3D3 #FFFFE0

Here's also a screenshot of the matplotlib figure

And the plotly converted figure before the change (unexpected light blue grey background)

And after the change (expected white background)