Plotly add_hline() doesn't work with simple_white template

Question

I was attempting to add a horizontal line to a Plotly graph using Plotly's add_hline(). It works perfectly fine so long as I do not use the simple_white template. The MWE is here:

import plotly.express as px

df = px.data.iris()
fig = px.scatter(df, x="petal_length", y="petal_width")
fig.add_hline(y=0.9, line_dash='dash', line_color='Red')
fig.update_layout(width=400, height=400,
                  # template='simple_white'
                  )
fig.show()

Without the template='simple_white' line, the add_hline() works as expected. With that template though, and with no other changes, the plot is the same but without the horizontal line.

Answer 1

I can't say why this happens, but you can get around it by explicitly specifying line_width and opacity for the horizontal line, without messing with the template:

fig.add_hline(y=0.9, opacity=1, line_width=2, line_dash='dash', line_color='Red')

If you only specify line width, the line looks washed out, possibly some of the themes (seaborn and ggplot2 in addition to plotly_white) change these two parameters?