px: facet_row_spacing=0 / facet_col_spacing=0 are ignored #5880

Passing facet_row_spacing=0 or facet_col_spacing=0 to a Plotly Express function has no effect: the default gap (0.03 / 0.07 between rows, 0.02 between columns) is used instead. The docstring allows any float between 0 and 1, and make_subplots(horizontal_spacing=0, vertical_spacing=0) works fine.

import plotly.express as px

df = px.data.tips()
fig = px.scatter(df, x="total_bill", y="tip", facet_col="smoker", facet_col_spacing=0)
print(fig.layout.xaxis2.domain[0] - fig.layout.xaxis.domain[1])  # 0.02, expected 0

Same with px.imshow(..., facet_col=..., facet_col_spacing=0). Seen on 7.1.0 and current main. The cause is args.get("facet_row_spacing") or 0.07 (and the same pattern for the other defaults) in init_figure, which treats 0 as unset. I have a fix ready.

Disclosure: I found and investigated this with help from an AI assistant (Claude).