feat(dp-forecast): warn when selected t0s returned no forecast data

Author: braddf

src/dataplatform/forecast/main.py

@@ -96,6 +96,8 @@ def init_session_state():
         st.session_state.merged_metrics_df = None
     if "fetch_time_stats" not in st.session_state:
         st.session_state.fetch_time_stats = ""
+    if "t0_coverage" not in st.session_state:
+        st.session_state.t0_coverage = None
     if "locked_params" not in st.session_state:
         st.session_state.locked_params = None
 
@@ -156,9 +158,38 @@ async def async_dp_forecast_page() -> None:
                     f"in `{fetch_duration:.2f}` seconds."
                 )
 
+                # Work out which requested t0s actually came back with data. A t0 that the
+                # Data Platform has no forecast for returns no rows (silently), so the chart's
+                # time axis only spans the t0s that did return — surface that here rather than
+                # leaving it looking like the axis is wrong.
+                requested_t0s = cfg.t0s or []
+                if requested_t0s and not df_forecast.empty:
+                    returned_t0s = set(df_forecast["initialization_timestamp_utc"])
+                    missing = [t0 for t0 in requested_t0s if t0 not in returned_t0s]
+                    st.session_state.t0_coverage = {
+                        "requested": len(requested_t0s),
+                        "returned": len(requested_t0s) - len(missing),
+                        "earliest_returned": (
+                            min(returned_t0s) if returned_t0s else None
+                        ),
+                    }
+                else:
+                    st.session_state.t0_coverage = None
+
         if st.session_state.fetch_time_stats:
             st.success(st.session_state.fetch_time_stats)
 
+        coverage = st.session_state.get("t0_coverage")
+        if coverage and coverage["returned"] < coverage["requested"]:
+            earliest = coverage["earliest_returned"]
+            earliest_str = earliest.strftime("%Y-%m-%d %H:%M") if earliest else "n/a"
+            st.warning(
+                f"Only `{coverage['returned']}` of `{coverage['requested']}` selected t0s "
+                f"returned forecast data (earliest with data: {earliest_str} UTC). "
+                "The time axis spans just those — the Data Platform returned nothing for the "
+                "rest."
+            )
+
         # Ensure we have data before trying to plot
         if (
             st.session_state.forecast_df is not None