Pin a chart to the last few runs of a flow

backend/tests/flow/test_dashboards.py

@@ -493,3 +493,24 @@ def test_a_flow_declaring_no_outputs_draws_no_stats():
     defn = results_dashboard(training_flow().model_copy(update={"outputs": []}))
 
     assert [w.type for w in defn.widgets] == ["chart"]
+
+
+def runs_chart(**runs) -> WidgetDef:
+    return WidgetDef(id="curves", type="chart", config={"source": "runs", "runs": runs})
+
+
+def test_a_chart_pinned_to_runs_reads_no_live_message():
+    """Its series is in the run tables; the engine has nothing to keep for it."""
+    widget = runs_chart(metric="study.loss", flow="study", latest=3)
+
+    assert widget.messages == []
+    assert widget.history_points == 0
+
+
+def test_a_chart_of_runs_must_say_which_runs_and_which_metric():
+    with pytest.raises(ValueError, match="metric"):
+        runs_chart(flow="study")
+    with pytest.raises(ValueError, match="a flow, a sweep, or run ids"):
+        runs_chart(metric="study.loss")
+    with pytest.raises(ValueError, match="between 1 and 5"):
+        runs_chart(metric="study.loss", flow="study", latest=9)