I'm aware of the issue, and that's why I have a "canvas_proportion" variable, which is used to compare with the processing screen proportion to determine the position of the dots

1. // coded by Ji Zhang (hope.zh@gmail.com)
2. // Sep-2013
3. JSONObject stopsJSON;
4. JSONArray stops;
5. Stop[] stations;
6. float latitude_mid;
7. float latitude_min = 360;
8. float latitude_max = -360;
9. float longitude_mid;
10. float longitude_min = 360;
11. float longitude_max = -360;
12. float delta_max;
13. float plotSize, halfPlotSize;
14. 
    
15. ///////////////////////////////////////////////////////////////////////////
16. void setup() {
17.   size(800, 500);
18.   plotSize = min(width, height);
19.   halfPlotSize = plotSize/2;
20.   stopsJSON = loadJSONObject("stops.json");
21.   stops = stopsJSON.getJSONArray("features");
22.   //println(stops.size());
23.   stations = new Stop[stops.size()];
24.   for (int i = 0; i < stops.size(); i++) {
25.     JSONObject stop_i = stops.getJSONObject(i);
26.     JSONObject stop_i_properties = stop_i.getJSONObject("properties");
27.     int stopId            = stop_i_properties.getInt("stopId");
28.     int stopNumber        = stop_i_properties.getInt("stopNumber");
29.     String stopName       = stop_i_properties.getString("stopName");
30.     String stopName_short = stop_i_properties.getString("stopNameShort");
31.     JSONObject stop_i_geometry = stop_i.getJSONObject("geometry");
32.     double stop_longitude = stop_i_geometry.getJSONArray("coordinates").getDouble(0);
33.     double stop_latitude  = stop_i_geometry.getJSONArray("coordinates").getDouble(1);
34. 
    
35.     if ( (float)stop_longitude < longitude_min ) {
36.       longitude_min = (float)stop_longitude ;
37.     }
38.     else if ( (float)stop_longitude > longitude_max ) {
39.       longitude_max = (float)stop_longitude ;
40.     }
41. 
    
42.     if ( (float)stop_latitude < latitude_min ) {
43.       latitude_min = (float)stop_latitude ;
44.     }
45.     else if ( (float)stop_latitude > latitude_max ) {
46.       latitude_max = (float)stop_latitude ;
47.     }
48. 
    
49.     println("_________________________");
50.     println("stop sequence: " + i);
51.     println("stopId: " + stopId);   
52.     println("stopNumber: " + stopNumber);
53.     println("stopName: " + stopName);
54.     println("stopName_short: " + stopName_short);
55.     println("stop_longitude: " + stop_longitude);
56.     println("stop_latitude: " + stop_latitude);
57. 
    
58.     stations[i] = new Stop();
59.     stations[i].stopId = stopId;
60.     stations[i].stopNumber = stopNumber;
61.     stations[i].stopName = stopName;
62.     stations[i].stopName_short = stopName_short;
63.     stations[i].stop_longitude = stop_longitude;
64.     stations[i].stop_latitude = stop_latitude;
65.   }
66. 
    
67.   latitude_mid = (latitude_max + latitude_min)/2;
68.   longitude_mid = (longitude_max + longitude_min)/2;
69.   delta_max = max((latitude_max - latitude_min), (longitude_max-longitude_min));
70.   println("longitude_min: " + longitude_min);
71.   println("longitude_max: " + longitude_max);
72.   println("latitude_min: "  + latitude_min);
73.   println("latitude_max: "  + latitude_max);
74.   println("longitude_max: " + longitude_max);
75. }
76. 
    
77. ///////////////////////////////////////////////////////////////////////////
78. void draw() {
79.   background(0);
80.   fill(96);
81.   noStroke();
82.   rect((width-plotSize)/2, (height-plotSize)/2, plotSize, plotSize);
83.  
84.   strokeWeight(2);
85.   stroke(255);
86.   for (Stop stop : stations) {
87.     stop.drawStop();
88.   }
89. }
90. 
    
91. class Stop {
92.   int stopId;
93.   int stopNumber;
94.   String stopName;
95.   String stopName_short;
96.   double stop_longitude;
97.   double stop_latitude;
98. 
    
99.   void Stop () {
100.   }
101. 
     
102.   void drawStop () {
103.     float ptX = (float)(stop_longitude - longitude_mid) / delta_max;
104.     ptX = map(ptX, -0.5, 0.5, -halfPlotSize, halfPlotSize);
105.     float ptY = (float)(latitude_mid - stop_latitude) / delta_max;
106.     ptY = map(ptY, -0.5, 0.5, -halfPlotSize, halfPlotSize);
107.     point(ptX + width/2, ptY + height/2);
108.   }
109. }