Use Pandas index in Plotly Express

Reference: https://plot.ly/python/px-arguments/#using-the-index-of-a-dataframe

You can pass the index as reference explicitly.

So in your case, this would be:

import plotly.express as px
iris = px.data.iris()
fig = px.scatter(iris, x=iris.index, y="sepal_length")
fig.show()

—

BONUS QUESTION: what if iris has a pd.MultiIndex?

Use pd.MultiIndex.get_level_values.

import plotly.express as px

# dummy example for multiindex
iris = px.data.iris().set_index(['species', 'species_id', iris.index])

fig = px.scatter(
   iris, 
   x=iris.index.get_level_values(2), 
   y="sepal_length"
)

fig.show()