Plotly, same scale for x and y axis

You can assign same length for height and width in your layout. Here is an example:

layout = Layout(
    xaxis=XAxis(
       range=[-150, 150],
       showgrid=True,
       zeroline=True,
       showline=True,
       gridcolor='#bdbdbd',
       gridwidth=2,
       zerolinecolor='#969696',
       zerolinewidth=4,
       linecolor='#636363',
       linewidth=6
    ),
    yaxis=YAxis(
        range=[-150,150],
        showgrid=True,
        zeroline=True,
        showline=True,
        gridcolor='#bdbdbd',
        gridwidth=2,
        zerolinecolor='#969696',
        zerolinewidth=4,
        linecolor='#636363',
        linewidth=6
   ),
   height=600,
   width=600,
)

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.