diff --git a/src/api.rs b/src/api.rs
index 48ed81b5..c78cd211 100644
--- a/src/api.rs
+++ b/src/api.rs
@@ -409,7 +409,13 @@ pub async fn metrics(
         // gone by the time the rows are built. Additive, so a theme unaware of it
         // continues to work.
         let (ping, loss) = if want_ping { app.db.ping_records(id, span)? } else { (vec![], json!({})) };
-        anyhow::Ok(json!({"metrics": metrics, "ping": ping, "probes": probes, "loss": loss}))
+        // `step` is the seconds each point spans, against which a row's `minutes`
+        // is read; inferred from the stamps instead, it would be missing for a
+        // window holding a single point. The last point is the bucket still in
+        // progress and holds only the minutes elapsed.
+        anyhow::Ok(json!({
+            "metrics": metrics, "ping": ping, "probes": probes, "loss": loss, "step": span.step,
+        }))
     })
     .await;
     match built.map_err(|e| anyhow::anyhow!(e)).and_then(|r| r) {
@@ -418,7 +424,8 @@ pub async fn metrics(
     }
 }
 
-/// The window a chart request is answered with, `hours` wide.
+/// The window a chart request is answered with: `hours` back from `now`,
+/// widened to the point boundary at or before that.
 ///
 /// The width is capped by the retention window alone, the same for every
 /// caller, because no width costs more than the week: up to `DETAIL_DAYS` a
@@ -450,7 +457,10 @@ fn span(hours: i64, points: Option<i64>, now: i64) -> db::Span {
     // evenly would keep the finer step and exceed it. `i64::div_ceil` is still
     // unstable, and both operands are positive here.
     let step = unit * ((hours * 3_600 / unit + budget - 1) / budget).max(1);
-    db::Span { since: now - hours * 3_600, step, hourly }
+    // Rows are bucketed by `ts / step` from the epoch, and a window opening
+    // partway through a bucket would leave its first point short of the
+    // `step / 60` minutes a whole one holds.
+    db::Span { since: (now - hours * 3_600).div_euclid(step) * step, step, hourly }
 }
 
 /// Guards a per-node read: the panel sees everything, while the public page sees
@@ -2448,11 +2458,15 @@ mod tests {
             );
             // Thinned, but neither empty nor reaching outside the window.
             assert!(!metrics.is_empty() && !ping.is_empty(), "{hours}h returned nothing");
-            // A bucket the window opens partway through begins before it.
             assert!(
-                metrics.iter().all(|m| m["ts"].as_i64().unwrap() >= span.since - span.step),
+                metrics.iter().all(|m| m["ts"].as_i64().unwrap() >= span.since),
                 "{hours}h reached back too far"
             );
+            // Begun on a point boundary, so the first point is a whole one where
+            // the history reaches past it: this node reported every minute.
+            if hours < 720 {
+                assert_eq!(metrics[0]["minutes"], span.step / 60, "{hours}h opened partway through a point");
+            }
         }
 
         // A day returns every minute it holds: thinning exists only for what the
@@ -3427,6 +3441,7 @@ mod tests {
         };
         let rows =
             |body: &[u8]| serde_json::from_slice::<Value>(body).unwrap()["metrics"].as_array().unwrap().len();
+        let step = |body: &[u8]| serde_json::from_slice::<Value>(body).unwrap()["step"].clone();
 
         // Before the first rollup nothing is folded, and a window past the week
         // reads the newest week of minute rows rather than every one.
@@ -3437,11 +3452,14 @@ mod tests {
         app.db.set("retention_days", "7").unwrap();
         let week = ask(168).await;
         assert_eq!(rows(&week), 8, "a week reaches back seven days");
+        assert_eq!(step(&week), 420, "in seven-minute points, a week of minutes at the 1,440-point budget");
         assert_eq!(ask(2_160).await, week, "a window past the retention window is narrowed to it");
 
         // Past the week, from the hourly tier and the minute rows after it.
         app.db.set("retention_days", "30").unwrap();
-        assert_eq!(rows(&ask(2_160).await), 30, "a month reaches back thirty days");
+        let month = ask(2_160).await;
+        assert_eq!(rows(&month), 30, "a month reaches back thirty days");
+        assert_eq!(step(&month), 3_600, "in whole hours, as no hourly row may straddle two points");
     }
 
     #[tokio::test]