Display 'ERR' instead of '-' for unset metrics
This makes it more clear this is an error condition, and something is
wrong in the schema.
@@ -2225,7 +2225,7 @@ class Chip:
value = self.get('metric', step, index, metric, 'real')
if value is None:
- value = '-'
+ value = 'ERR'
else:
value = str(value)
