Trip distance by means of transport: air and rail

Closes: #148

agenda/types.py

@@ -2,6 +2,7 @@
 
 import datetime
 import typing
+from collections import Counter
 from dataclasses import dataclass, field
 
 from pycountry.db import Country
@@ -141,6 +142,25 @@ class Trip:
             else None
         )
 
+    def distances_by_transport_type(self) -> list[tuple[str, float]]:
+        """Calculate the total distance travelled for each type of transport.
+
+        Any travel item with a missing or None 'distance' field is ignored.
+        """
+        transport_distances: Counter[float] = Counter()
+
+        for item in self.travel:
+            distance = item.get("distance")
+            if distance:
+                transport_type = item.get("type", "unknown")
+                transport_distances[transport_type] += distance
+
+        return list(transport_distances.items())
+
+
+# Example usage:
+# You would call the function with your travel list here to get the results.
+
 
 @dataclass
 class Holiday: