secondary_y creates a small spacing on the right

It looks like secondary_y would cause the chart to render a small chunk of whitespace to the right of it.

With secondary_y:

import plotly.graph_objects as go
from plotly.subplots import make_subplots


fig = make_subplots(specs=[[{'secondary_y': True}]])

fig.add_trace(go.Scatter(x=[1, 2, 3], y=[40, 50, 60], name='yaxis data'), secondary_y=False)
fig.add_trace(go.Scatter(x=[2, 3, 4], y=[4, 5, 6], name='yaxis2 data'), secondary_y=True)

fig.update_layout(legend={'y': 1, 'x': 0}, margin={'t': 0, 'b': 0, 'l': 0, 'r': 0})
fig.show()

Without secondary_y:

import plotly.graph_objects as go
from plotly.subplots import make_subplots


fig = make_subplots()

fig.add_trace(go.Scatter(x=[1, 2, 3], y=[40, 50, 60], name='yaxis data'))
fig.add_trace(go.Scatter(x=[2, 3, 4], y=[4, 5, 6], name='yaxis2 data'))

fig.update_layout(legend={'y': 1, 'x': 0}, margin={'t': 0, 'b': 0, 'l': 0, 'r': 0})
fig.show()

Start by running the two Plotly Python examples in the issue and compare the rendered chart widths with and without secondary_y. Trace the subplot and layout handling involved in secondary_y; done means the extra right-side whitespace is gone while both y-axes and the displayed traces remain correct.