Raise exception in multiple data groups case

Author: yozhikoff

lib/matplotlib/axes/_base.py

@@ -293,6 +293,10 @@ def __call__(self, *args, data=None, **kwargs):
                     replaced[label_namer_idx], args[label_namer_idx])
             args = replaced
 
+        if len(args) >= 4 and not cbook.is_scalar_or_string(kwargs["label"]):
+            raise ValueError("plot() with multiple groups of data (i.e., pairs "
+                             "of x and y) does not support multiple labels")
+
         # Repeatedly grab (x, y) or (x, y, format) from the front of args and
         # massage them into arguments to plot() or fill().