Step 1. Compare stores' sales dynamics.
Display sales charts for all stores; the minimum period is 1 year.
Compare the dynamics of the performances of stores: revenue, number of purchases, average amount of purchases, units per transaction, and so on. Clustering algorithms can help detect stores with similar performances.
When there are few stores, one can even find the stores with similar dynamics in a visual way.
To discover hidden trends, one should analyze the sales dynamics for several years. As mentioned above, hidden trends are cases when the store is slightly but steadily growing or "falling".
Example 3 at the beginning of the article (about how the new residential complex affected sales during the pilot) shows why considering trends is crucial for the pilot evaluation: