Mapnik's aspect_fix_mode is both wonderful and evil. This page explains why.
Background
Mapnik uses a bounding box (aka. envelope or bounding rectangle) to decide what data to query and where to place it on a given map.
A bounding box (BBOX) is made up of 2 x,y coordinate pairs representing the lower left corner and the upper right and is expressed as minx,miny,maxx,maxy. So a BBOX that represents the entire world is -180,-90,180,90 if we are speaking in long/lat degrees. The coordinate values are specific to the spatial reference system you are using, so another common BBOX is the global extents in web mercator: -20037508.342789244,-20037508.342789244,20037508.342789244,20037508.342789244. Curious how these were calculated? See this code:
their absolute values are measuring the exact length in meters of a path, along the equatorial parallel, on the reference WGS84 geoid modelizing the Earth surface at average sea level, from any longitude to its antipodic one (i.e. a path along 180 degrees of longitude, or the half of the average equatorial circumference), assuming that this equatorial parallel is a perfect circle (this is not exactly the case for the true Earth, but it is true for the reference geoid used in our web Mercator projection).
With common tiling schemes it is easy to know the BBOX based on a given zoom level and x/y tile id and to know the reverse. Learn more at http://www.maptiler.org/google-maps-coordinates-tile-bounds-projection/ and see some code that does this at https://github.com/mapbox/node-sphericalmercator.
Are BBOX's square?
They can be, but often are not. If you look back above you'll notice that the mercator bounding box is square, which makes sense because the web mercator projection specifically excludes latitudes ±85.05113° to make web mercator work well in a square tiling scheme
Are rendered map images square?
They can be, but often are not. Mapnik allows the user to control the width and height of a rendered image. While tiling renderers usually only allow images that are 256x256, Mapnik allows any width or height greater than 0.
Aspect ratio of BBOX's and the rendered map
If your BBOX is square then (maxx-minx)/(maxy-miny) will equal 1 meaning your aspect ratio is 1:1. If your rendered map width is equal to its height then your map is also square with an aspect ratio of 1:1.
What happens when the aspect ratio of the BBOX or map are not 1:1?
Mapnik automatically adjusts the BBOX. So, the aspect ratio of the map width and height is calculated and used to grow the appropriate dimension of the BBOX so that the ratios match.
This default internally in Mapnik is called aspect_fix_mode.GROW_BBOX and is the default because of this line. The other possible behaviors are to shrink the BBOX, or grow or shrink the map dimensions. All the options can be see here. Whenever the map bbox is set or the image dimensions are modified this code is triggered to "fix" the aspect ratios.
How to change the defaults
In python:
Say we wanted to change the mode to ADJUST_BBOX_HEIGHT and we have a map in memory with the variable m:
C++:
m.set_aspect_fix_mode(mapnik::Map::ADJUST_BBOX_HEIGHT);
Python:
m.aspect_fix_mode = mapnik.aspect_fix_mode.ADJUST_BBOX_HEIGHT
For a python example of this in action see https://gist.github.com/andrewharvey/1290744
Javascript:
m.aspect_fix_mode = mapnik.Map.ADJUST_BBOX_HEIGHT;