I tried to overlay a 'threashold' line on a bar graph but didn't managed to have a clean graph.

Here is my pandas DataFrame:

import pandas as pd 
df = pd.DataFrame({'Nutriment' : ['Glucides (g)', 'Lipides (g)', 'Protéines (g)'],
      'ail' : [4, 0.056, 7.82],
      'basilic' : [0.05, 0.009, 0.42],
      'carotte' : [2.6, 0.1, 3.44],
      'citron vert' : [0.063, 0.0, 0.14]})

and this is what I tried to do :

from plotly.subplots import make_subplots
import plotly.graph_objects as go

fig = make_subplots()
fig.update_layout(xaxis2= {'anchor' : 'y', 'overlaying' : 'x', 'side' : 'bottom'})

ingredients = df.columns[1:]
clrs = {}
n = 100
for ingredient in ingredients : 
    clrs[ingredient] = 'rgb(0,'   str(n)   ','   str(n)   ')'
    n  = 40
    
                    
for ingredient in ingredients :
    fig.add_trace(go.Bar(x=df['Nutriment'], 
                         y=df[ingredient], 
                         name=ingredient, 
                         marker= {'color' : clrs[ingredient]}, xaxis = 'x2'))
    
fig.add_trace(go.Scatter(x=[0,1], y=[4.5, 4.5], name='Apport recommandé en glucides (min)', line_color='#ff0040'))
fig.add_trace(go.Scatter(x=[1,2], y=[1.9, 1.9], name='Apport recommandé en lipides (min)', line_color='#00ff00'))
fig.add_trace(go.Scatter(x=[2,3], y=[1, 1], name='Apport recommandé en protéines (min)', line_color='#0000f0'))

fig.update_layout(width=800, height=500, title_text='Apports nutritionnels par portion', barmode='stack')