diff --git a/plotly/shapeannotation.py b/plotly/shapeannotation.py
index a2323ed02d..9f1e0f7682 100644
--- a/plotly/shapeannotation.py
+++ b/plotly/shapeannotation.py
@@ -4,7 +4,14 @@
 def _mean(x):
     if len(x) == 0:
         raise ValueError("x must have positive length")
-    return float(sum(x)) / len(x)
+    # Handle non-numeric values (e.g., date strings)
+    # For vline/hline, x0==x1 and y0==y1, so returning first value is correct
+    try:
+        return float(sum(x)) / len(x)
+    except TypeError:
+        # If sum fails (e.g., for date strings), return the first value
+        # This works because for vlines x0==x1, and for hlines y0==y1
+        return x[0]
 
 
 def _argmin(x):