Trying to assign to an un-named type

Hey Guys,

I'm trying to write a library for my Adafruit Bi-color 8x8 matrix so that I can release it for others to use. As it is a display and so involves drawing shapes and such, I thought a good idea would be to extend off of the Graphics object and some custom methods to that and return that as my modules API, like so:

// Define global constants
var LED_OFF = 0;
var LED_RED = 1;
var LED_YELLOW = 2;
var LED_GREEN = 3;

var HT16K33_BLINK_CMD = 0x80;
var HT16K33_BLINK_DISPLAYON = 0x01;
var HT16K33_BLINK_OFF = 0;
var HT16K33_BLINK_2HZ  = 1;
var HT16K33_BLINK_HALFHZ  = 3;
var HT16K33_BRIGHTNESS_CMD = 0xE0;
var HT16K33_ENABLE_OSCILLATOR_CMD = 0x21;

var exports = {};
exports.connect = function(i2c, address, callback){

  // Private methods
  function drawPixel(x,y,col){
      
    // Handle rotation
    switch (rotation) {
    case 1:
      var t1 = x;
      x = 8 - y - 1;
      y = t1;
      break;
    case 2:
      x = 8 - x - 1;
      y = 8 - y - 1;
      break;
    case 3:
      var t2 = x;
      x = y;
      y = 8 - t2 - 1;
      break;
    }
    
    // Handle backwards wiring
    if(x < 4){
      x += 4;
    } else {
      x -= 4;
    }
    
    if(y < 4){
      y += 4;
    } else {
      y -= 4;
    }
    
    // We wamt colours to override
    // so turn the current pixel off by default
    // then set it's colour again
    displaybuffer[y] &= ~(1 << x) & ~(1 << (x+8));

    if (col == LED_GREEN) {
      displaybuffer[y] |= (1 << x) & ~(1 << (x+8));
    } else if (col == LED_RED) {
      displaybuffer[y] |= ~(1 << x) & (1 << (x+8));
    } else if (col == LED_YELLOW) {
      displaybuffer[y] |= (1 << x) | (1 << (x+8));
    }
      
  }
  
  function drawRect(x1,y1,x2,y2,col){
    for(var x=x1;x<=x2;x++){
      for(var y=y1;y<=y2;y++){
        drawPixel(x,y,col);
      }
    }
  }
  
  // Create buffer
  var display = Graphics.createCallback(8,8,8,{setPixel:drawPixel,fillRect:drawRect});
  var displaybuffer = new Uint16Array(8);
  var rotation = 0;
  
  display.setRotation = function(rot){
    rotation = rot;
  };
  
  display.getRotation = function(){
    return rotation;
  };
  
  display.setBlinkRate = function(b) {
    if (b > 3) b = 0; // turn off if not sure
    i2c.writeTo(address, HT16K33_BLINK_CMD | HT16K33_BLINK_DISPLAYON | (b << 1));
  };
    
  display.setBrightness = function(b) {
    if (b > 15) b = 15; // Don't go over max
    i2c.writeTo(address, HT16K33_BRIGHTNESS_CMD | b);
  };
    
  display.clear = function(){
    for (var i=0; i<8; i++) {
      displaybuffer[i] = 0;
    }
    display.write();
  };
  
  display.write = function(){
    var tmp = new Uint8Array(17);
    tmp[0] = 0x00;
    for (var i=0; i<16; i+=2) {
      tmp[i+1] = displaybuffer[i/2] & 0xFF;
      tmp[i+2] = displaybuffer[i/2] >> 8;
    }
    i2c.writeTo(address, tmp);
  };
  
  // Turn on oscillator
  i2c.writeTo(address, HT16K33_ENABLE_OSCILLATOR_CMD);
  
  // Set blink rate
  display.setBlinkRate(HT16K33_BLINK_OFF);
  
  // Set to full brightness
  display.setBrightness(15);
  
  // Trigger callback
  if (callback!==undefined){
    setTimeout(callback, 100);
  }
  
  return display;
};


// Setup I2C
I2C2.setup({ scl: B10, sda: B11 });

var matrix = exports.connect(I2C2, 0x70, function(){
  
  var counter = 1;
  setInterval(function(){
    
    matrix.clear();
    
    matrix.setRotation(counter - 1);
    matrix.setColor(counter);
    matrix.fillRect(0,0,7,1);
    matrix.fillRect(0,3,7,4);
    matrix.fillRect(0,6,7,7);
    matrix.fillRect(0,0,1,7);
    
    matrix.write();
    
    counter = (counter == 3) ? 1 : counter + 1;
    
  }, 2000);
  
});

The problem is, I keep getting errors when I try to add additional methods to my display object. If I extend the Graphics class via prototype then everything works, however I don't won't my methods available to all Graphics objects, just ones returned from my module. Anyone know what I'm doing wrong (I'm sure I saw @Gordon do this kind of thing in the 123-LED demo video).

Matt