diff --git a/affordances/officer/core/starting-an-app.md b/affordances/officer/core/starting-an-app.md
index e550384..e84c087 100644
--- a/affordances/officer/core/starting-an-app.md
+++ b/affordances/officer/core/starting-an-app.md
@@ -59,6 +59,24 @@ they exist so the same argument is not had twice.
 Drop the template's git history and start your own unless told otherwise: it is a starting point, not an
 upstream to track.
 
+## Data to build against
+
+There is mock data, so a dashboard has something real to show:
+
+```bash
+git clone git@gitea.officer.dev:templates/sample-data.git
+```
+
+`sales` — 6,402 daily orders over two years, with a Q4 lift, a summer dip, quiet weekends and one region
+pulling ahead in year two. `subscriptions` — 480 accounts where "active" is a calculation over start and
+cancel dates rather than a column, and small plans churn far more than large ones.
+
+Both come as `.xlsx` and `.csv`, identical data. **The CSV needs no parser**, so prefer it unless reading
+the spreadsheet is the point. Its `README.md` has every column.
+
+Use this rather than inventing rows. Generated data with no shape in it produces a chart that proves
+nothing, and that is what gets built when an agent makes numbers up.
+
 ## Running it — always as a background job
 
 ```bash