Bug with plot_prediction() when forecast_length=1

[valer1435]

When forecast_length=1 model.plot_prediction() doesn't show predict

data df_daily.xlsx notebook to reproduce https://colab.research.google.com/drive/1iKOufLVJYYf7xTJt0yDklqB7iiTMP6AA?usp=sharing

PS need to check is it common for univariate case too