Avoid division by zero in hodograph production

lib/xmet/hodograph.py

@@ -33,6 +33,9 @@ class Hodograph():
             box_height = max_y - min_y
             box        = max(box_width, box_height) * 1.5
 
+            if box == 0:
+                box = 1.5
+
             #
             # Ensure the data points are centered within the hodograph
             # viewbox.