I have used the code from Mapnik python tutorial. Now I want to use zoom_to_box to zoom to certain extent. I want to zoom to Box2d(-180.0, 0.0, 0.0, 90.0), however, after appied zoom_to_box, I saw the map envelope is Box2d(-180.0,-45.0,0.0,135.0), so what do I miss here?
The testing shapefile...