# myopic optimization [2020, 2030, 2040, 2050] since H2 retrofit is allowed from 2030 on, code gets easier
year = 2040 
CH4_per_H2 = 0.6
# already retrofitted
h2_retrofitted_fixed_i = n.links[
    (n.links.carrier == "H2 pipeline retrofitted")
    & (n.links.build_year != year)
].index
h2_retrofitted = n.links[(n.links.carrier=="H2 pipeline retrofitted") & 
                         (n.links.build_year == year)].index

potential = n.links.loc[h2_retrofitted].p_nom_max
already_retro = n.links.loc[h2_retrofitted_fixed_i].p_nom

# gas pipelines
gas_index = potential.index.str[:-5]
gas_index = gas_index.str.replace("H2 pipeline retrofitted", "gas pipeline")
gas_cap = n.links.loc[gas_index].p_nom*CH4_per_H2

# plotting
potential.index = potential.index.str.replace("H2 pipeline retrofitted ", "")
potential.index = potential.index.str[:-5]
already_retro.index = already_retro.index.str.replace("H2 pipeline retrofitted ", "")
already_retro.index = already_retro.index.str[:-5]
gas_cap.index = gas_cap.index.str.replace("gas pipeline ","")

df = pd.concat([-gas_cap, potential, already_retro], axis=1)
df = df.fillna(0)
df.columns = ['gas', 'H2 retro potential', 'H2 retro']
df.plot.bar(stacked=True)