Whenever you add a hline/vline to a plot it should display it at the given position. However, when the figure is made with make_subplots and is not yet populated with data, it never shows the line.

importplotly.graph_objectsasgofromplotly.subplotsimportmake_subplotsforname,figin {"go.Figure":go.Figure(),"make_subplots":make_subplots(rows=1,cols=1)}.items():fig.add_hline(y=0.5)# Add a trace after the linefig.add_trace(go.Scatter(x=[1,2],y=[0,1]))fig.update_layout(title=name)fig.show()

pip list

Package Version

asttokens 3.0.0
attrs 25.3.0
colorama 0.4.6
comm 0.2.2
debugpy 1.8.13
decorator 5.2.1
executing 2.2.0
fastjsonschema 2.21.1
ipykernel 6.29.5
ipython 9.0.2
ipython_pygments_lexers 1.1.1
jedi 0.19.2
jsonschema 4.23.0
jsonschema-specifications 2024.10.1
jupyter_client 8.6.3
jupyter_core 5.7.2
matplotlib-inline 0.1.7
narwhals 1.31.0
nbformat 5.10.4
nest-asyncio 1.6.0
packaging 24.2
parso 0.8.4
pip 24.3.1
platformdirs 4.3.7
plotly 6.0.1
prompt_toolkit 3.0.50
psutil 7.0.0
pure_eval 0.2.3
Pygments 2.19.1
python-dateutil 2.9.0.post0
pywin32 310
pyzmq 26.3.0
referencing 0.36.2
rpds-py 0.23.1
six 1.17.0
stack-data 0.6.3
tornado 6.4.2
traitlets 5.14.3
typing_extensions 4.12.2
wcwidth 0.2.13