fix default value for stacked

before default was de-facto true which was against the intention

src/account_diagram.py

@@ -130,7 +130,7 @@ def draw_line_diagram(data: Categorisation,
                           Tuple[float, float, float],
                           str]] = None,
                       filename: str = None,
-                      stacked: str = "plot",
+                      stacked: str = False,
                       ymax = None):
     """Draw a line diagram with the money movements.