Sanitize pyarrow-backed timestamp columns to ISO8601 string

altair/utils/core.py

@@ -366,7 +366,7 @@ def to_list_if_array(val):
             # https://pandas.io/docs/user_guide/boolean.html
             col = df[col_name].astype(object)
             df[col_name] = col.where(col.notnull(), None)
-        elif dtype_name.startswith("datetime"):
+        elif dtype_name.startswith("datetime") or dtype_name.startswith("timestamp"):
             # Convert datetimes to strings. This needs to be a full ISO string
             # with time, which is why we cannot use ``col.astype(str)``.
             # This is because Javascript parses date-only times in UTC, but