This script is designed to provide a simple way to add a 'quick and dirty' scale bar to a Mapnik map using PIL. It works out how long it wants to be based upon the size of the map, and draws itself to ...

Now add a map item, the scale bar map property now shows 'Map 1' but still it's not resized to match the linked map (only 0 is shown) Afaics, you'd need to play with the scalebar units parameter ...