Same scale for x and y axis

Finally, this feature is implemented.

layout = go.Layout(yaxis=dict(scaleanchor="x", scaleratio=1))

Update: in new versions of plotly, use the following:

fig.update_yaxes(
    scaleanchor="x",
    scaleratio=1,
  )

See example here https://plot.ly/python/axes/#fixed-ratio-axes.