draw one diagonal from bottom up

draw-diagonal.js

var drawDiagonal = function(x,y,color) {

 ctx.fillStyle = color;
 ctx.fillRect(x, y, pixelSize, pixelSize);
 
 x = x + pixelSize;
 y = y - pixelSize;
 
 if (x >= 0 && y >= 0) {
  drawDiagonal(x,y);
 }

};