diff --git a/grafana/dashboard.py b/grafana/dashboard.py
index ab50b74..c7bf19f 100644
--- a/grafana/dashboard.py
+++ b/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