Al final aquello de divide y vencerás ha funcionado. Sólo cambiando el javascript con las coordenadas de Zaragoza… ¡tachán!

El javascript:

// THE GRID
var theGrid = new Object();
theGrid.latN = 41.69445;
theGrid.latS = 41.623399;
theGrid.lonW = -0.944138;
theGrid.lonE = -0.834618;
theGrid.SW = new google.maps.LatLng(theGrid.latS,theGrid.lonW);
theGrid.NE = new google.maps.LatLng(theGrid.latN,theGrid.lonE);

theGrid.bounds = new google.maps.LatLngBounds(theGrid.SW, theGrid.NE);
theGrid.cellSize = 250; // meters
theGrid.distance = 1; // number of cells square we use to calculate occupations
// END THE GRID

var city = new google.maps.LatLng("41.655553","-0.887806");

var APIurl = "http://api.citybik.es/bizi.json?callback=?";