Set axis linecolor to black by default 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.