Set barmode="overlay" in histograms by default to avoid misunderstandings

[pierre-eliep-met]

Reopening of #1865, as asked in the comment.

By default, histograms are not overlayed, which leads to misinterpretation :

import numpy as np
import pandas as pd
import plotly.express as px

x0 = np.random.randn(250)
# Add 1 to shift the mean of the Gaussian distribution
x1 = np.random.randn(250) + 1

df = pd.DataFrame(dict(
    series=np.concatenate((["a"] * len(x0), ["b"] * len(x1))), 
    data=np.concatenate((x0, x1))
))

px.histogram(df, x="data", color="series")

Makes think that there is much more points in red distribution :

While overlying clearly shows that it is just shifted

px.histogram(df, x="data", color="series", barmode="overlay")

This leads to misunderstanding the data shape.

[gvwilson]

@archmoj is this a plotly.py issue or should I move it to plotly.js?