use "main" as the default legend name

src/plot.py

@@ -306,7 +306,7 @@ def plot(
         info(f"plotting graph {i}")
 
         line = _plot_single(g, i, ax)
-        legend = g.get("legend", None)
+        legend = g.get("legend", "main" if "name" in g else None)
         if legend is not None:
             debug(f"adding legend of graph {i}")
             if legend not in legends: