Vertical line on barplot

I have a barplot and I want to plot a vertical line at x =3:

go.Bar(x=[1, 2, 3, 4],
y=[‘aa’, ‘bb’, ‘cc’, ‘dd’],

What value should have y0 and y1 from from property ‘shapes’ since y does not have numeric values:

‘shapes’: [
‘type’: ‘line’,
‘xref’: ‘x’,
‘yref’: ‘y’,
‘x0’: 3,
‘y0’: ?,
‘x1’: 3,
‘y1’: ?}

Moved to API - Python section for better relevance and answers. :slight_smile: