modified trendline formatting

[emmanuelle]

Closes #1984.
For example

import plotly.express as px

df = px.data.tips()
fig = px.scatter(df, x="total_bill", y=1.e-6 * df.tip, trendline="ols")
fig.show()

Master:

Now:

Existing examples should not be modified, I checked with the doc example of trendline.

[nicolaskruchten]

if existing examples are not impacted, then this is a clear win :)

💃