import plotly.express as px
import plotly.graph_objects as go
import numpy as np
df = px.data.iris()
fig = px.scatter(df, x="petal_length", y="petal_width")
fig.add_traces(go.Scatter(y=np.arange(1, 7), mode="lines+markers", yaxis='y2'))
fig.add_hline(y=2, line_dash='dash', line_color='Red', yref='y2')
fig.update_layout(
 yaxis2=dict(
 title="yaxis2 title",
 overlaying="y",
 side="right",
 )
)
fig.show()