Add support for overriding x tick with non arithmetic progression values

[robertoffmoura]

Hi,
This change adds support for specifying tickvals and ticktext when the values don't follow an arithmetic progression, i.e., they're not evenly spaced.

Code PR

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• I have added tests or modified existing tests.
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• I have added a CHANGELOG entry if changing anything substantial.
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[robertoffmoura]

Hi @emilykl,
This adds support for x ticks that don't follow an arithmetic progression when converting matplotlib figures to plotly.

Currently, the conversion ignores custom x tick values specified in the matplotlib figure.

See the snippet below, which prints None before this change:

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

xticks = [0.01, 0.53, 0.75]  # non-arithmetic progression

fig = plt.figure()
plt.plot([0, 1], [0, 1])
plt.xticks(xticks)
fig.savefig("matplotlib.png")

p = tls.mpl_to_plotly(plt.gcf())
print(p.layout.xaxis.tickvals) # (0.01, 0.53, 0.75)
p.write_image("plotly.png")

Here's a screenshot of the matplotlib figure with custom x tick values:

Before this change, the plotly conversion ignores the 0.75 x tick value.

After this change, the converted plot correctly shows the 0.75 x tick value.