fix: plot monthly demand

Author: phumthep

src/pownet/core/output.py

@@ -145,6 +145,7 @@ def get_daily_demand(self, input_demand: pd.DataFrame) -> pd.Series:
 
     def get_monthly_demand(self, input_demand: pd.DataFrame) -> pd.Series:
         monthly_demand = self.get_hourly_demand(input_demand)
+        monthly_demand = monthly_demand.to_frame()
         monthly_demand["month"] = self.dates["date"].dt.to_period("M")
         monthly_demand = monthly_demand.groupby("month").sum()
         monthly_demand.index = monthly_demand.index.strftime("%b")