[BUG] plot_hierarchical_predictions_gap is not general enough

[AzulGarza]

The following line is not general. Not all datasets contain the unique_id "['Total']".

hierarchicalforecast/hierarchicalforecast/utils.py

Lines 246 to 248 in 0a2d6c7

	if 'y' in Y_df.columns:
	y_plot = Y_df[Y_df.index =="['Total']"].y.values
	plt.plot(horizon_dates, y_plot, label='True')

The total unique_id could be recovered using tags.