Grafana: a panel querying Loki and Prometheus both is Mixed, so Downloads works (#226)

Each panel took its first query's data source, Loki for Downloads, whose at-once and torrent
lines are Prometheus's; Grafana runs another data source's query only on a Mixed panel.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

grafana/dashboard.py

@@ -51,8 +51,12 @@ pid = 0
 def panel(kind, title, w, h, x, targets, **extra):
     global pid
     pid += 1
+    sources = {t.get("datasource", LOKI)["uid"] for t in targets}
+    # A panel with queries on more than one data source must be Mixed, or Grafana runs them all on
+    # the panel's and Downloads, Loki's and Prometheus's, failed (#226).
+    ds = {"type": "datasource", "uid": "-- Mixed --"} if len(sources) > 1 else targets[0].get("datasource", LOKI)
     p = {"id": pid, "type": kind, "title": title, "gridPos": {"x": x, "y": y, "w": w, "h": h},
-         "datasource": targets[0].get("datasource", LOKI), "targets": targets}
+         "datasource": ds, "targets": targets}
     p.update(extra)
     panels.append(p)
     return p