How to expand bars over the month on the x-axis while being the same width? - dataframe

Question

for i in range(len(basin)):
prefix = "URL here"
state = "OR"
basin_name = basin[i]
df_orig = pd.read_csv(f"{prefix}/{basin_name}.csv", index_col=0)
#---create date x-index
curr_wy_date_rng = pd.date_range(
start=dt(curr_wy-1, 10, 1),
end=dt(curr_wy, 9, 30),
freq="D",
)
if not calendar.isleap(curr_wy):
print("dropping leap day")
df_orig.drop(["02-29"], inplace=True)
use_cols = ["Median ('91-'20)", f"{curr_wy}"]
df = pd.DataFrame(data=df_orig[use_cols].copy())
df.index = curr_wy_date_rng
#--create EOM percent of median values-------------------------------------
curr_wy_month_rng = pd.date_range(
start=dt(curr_wy-1, 10, 1),
end=dt(curr_wy, 6, 30),
freq="M",
)
df_monthly_prec = pd.DataFrame(data=df_monthly_basin[basin[i]].copy())
df_monthly_prec.index = curr_wy_month_rng
df_monthly = df.groupby(pd.Grouper(freq="M")).max()
df_monthly["date"] = df_monthly.index
df_monthly["wy_date"] = df_monthly["date"].apply(lambda x: cal_to_wy(x))
df_monthly.index = pd.to_datetime(df_monthly["wy_date"])
df_monthly.index = df_monthly["date"]
df_monthly["month"] = df_monthly["date"].apply(
lambda x: calendar.month_abbr[x.month]
)
df_monthly["wy"] = df_monthly["wy_date"].apply(lambda x: x.year)
df_monthly.sort_values(by="wy_date", axis=0, inplace=True)
df_monthly.drop(
columns=[i for i in df_monthly.columns if "date" in i], inplace=True
)
# df_monthly.index = df_monthly['month']
df_merge = pd.merge(df_monthly,df_monthly_prec,how='inner', left_index=True, right_index=True)
#---Subplots---------------------------------------------------------------
fig, ax = plt.subplots(figsize=(8,4))
ax.plot(df_merge.index, df_merge["Median ('91-'20)"], color="green", linewidth="1", linestyle="dashed", label = 'Median Snowpack')
ax.plot(df_merge.index, df_merge[f'{curr_wy}'], color='red', linewidth='2',label='WY Current')
#------Seting x-axis range to expand bar width for ax2
ax.bar(df_merge.index,df_merge[basin[i]], color = 'blue', label = 'Monthly %')
#n = n + 1
#--format chart
ax.set_title(chart_name[w], fontweight = 'bold')
w = w + 1
ax.set_ylabel("Basin Precipitation Index")
ax.set_yticklabels([])
ax.margins(x=0)
ax.legend()
#plt.xlim(0,9)
#---Setting date format
ax.xaxis.set_major_locator(mdates.MonthLocator())
ax.xaxis.set_major_formatter(mdates.DateFormatter('%b'))
#---EXPORT
plt.show()
End result desired: Plotting both the monthly dataframe (df_monthly_prec) with the daily dataframe charting only monthly values (df_monthly). The bars for the monthly DataFrame should ideally span the whole month on the chart.
I have tried creating a secondary axis, but had trouble aligning the times for the primary and secondary axes. Ideally, I would like to replace plotting df_monthly with df (showing all daily data instead of just the end-of-month values within the daily dataset).
Any assistance or pointers would be much appreciated! Apologies if additional clarification is needed.