I'm working with the CausalImpact package for predicting a counterfactual time series. From prior analysis, I know that the true counterfactual series ought to show a downward trend, but the package seems to predict horizontal trends.
In other words, if I were to draw a line through the average value of the predicted counterfactual, it'll be a horizontal line. However, I have the prior knowledge it actually should be a line with negative slope. The supplied covariates actually show a slight downward trend as well, but apparently it's not strong enough for the package to pick up on.
Question: Is there a way to incorporate prior knowledge about trend in the CausalImpact package? I'm vaguely aware that bsts allows for a custom model but I don't know if it's doable.