GML Coordinate Scaling Problem



I'm having a little bit of a coordinate problem.

I'm trying to use a 'heat map' to direct my unit objects. Head for hot sectors, avoid cold ones. It's not really working. They always head for the top left corner. I believe this is a scaling problem, but I don't know what to try next.

The relevant unit code is here:
// Heat!
var i,ii;
var xx = 0; // Target Coords
var yy = 0;
var heatstuff = 0; // Heat of current cell
var cell_size = controller.cell_size; // How big the cells are
var xcell = x div cell_size; // Our cell
var ycell = y div cell_size;
var side = controller.xcells; // How many cells in each direction
var top = controller.ycells;
var map = controller.heat_map; // The heat map we're using
var cell_checks = 5; // How many cells to check in each direction
var imin = max(0,xcell-cell_checks);
var imax = min(xcell+cell_checks,side);
var iimin = max(0,ycell-cell_checks)
var iimax = min(ycell+cell_checks,top);
var total_heat = ds_grid_get_sum(map,imin,iimin,imax,iimax); // Total heat of the area
if(total_heat == 0) // Making sure we don't get / by 0 errors
    total_heat = 1;
        heatstuff = ds_grid_get(map,i,ii) / total_heat;
        xx += ((i*cell_size)+(cell_size/2))*heatstuff;
        yy += ((ii*cell_size)+(cell_size/2))*heatstuff;
while(xx == x and yy == y) // Make sure our units have a direction
    xx = x+random(2)-1;
    yy = y+random(2)-1;
// Find the direction we want to go in
var dir = point_direction(x,y,xx,yy);
var len = 20;
// Set our goto coordinates
inv_x = x+lengthdir_x(len,dir);
inv_y = y+lengthdir_y(len,dir);
Here's a picture of what I mean. The units in question are the white ones, and the heat map is drawn over top.
Screenshot (22)_LI.jpg

I've tried not dividing by the total heat, and by finding the direction off of (0,0) instead of the unit's (x,y), but neither one of those gave a solution, just different, just as bad problems.

If you can help me out, thank you!