extension with plot_ts for timeseries

[waudinio27]

Notebook proposal

Title: Extension for timeseries notebooks to plot_ts for out of sample predictions

Why should this notebook be added to pymc-examples?

Because up to now all the notebooks related to timeseries and some of the examples that can be related to this only show how to fit the model and not how to make predictions after this. An idea to do so is the plot_ts function from arviz. This would help normal users like me a lot.

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