Only show a legend when the matplotlib figure has one

[robertoffmoura]

Converted matplotlib figures without legends unexpectedly show legends in plotly. mpl_to_plotly left layout.showlegend unset, and plotly.js defaults it to True, auto-naming unnamed traces, so every conversion without an explicit mpl legend (e.g. plt.plot(x, y) with no labels, boxplots) rendered with a legend of junk entries.

Fix:

• open_figure now sets showlegend=False by default; the legend is only enabled by open_legend when the matplotlib figure actually has one (existing behavior, unchanged)

Snippet to reproduce:

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

fig, ax = plt.subplots()
ax.plot([0, 1, 2, 3], [0, 1, 0, 1], "b", [0, 1, 2, 3], [1, 0, 1, 0], "r--")
fig.savefig("legend_mpl.png")

p = tls.mpl_to_plotly(fig)
p.write_image("legend_plotly.png")

matplotlib	plotly before	plotly after