Chart.prototype.drawTextCentered = function(context, text, x, y, font, color) {
  if (context.mozDrawText) {
    context.save();
    context.fillStyle = color;
    context.mozTextStyle = font;
    x -= 0.5 * context.mozMeasureText(text);
    context.translate(x, y);
    context.mozDrawText(text);
    context.restore();
  }
}

<canvas id="canvas"></canvas>
<script>
    var context = document.getElementById("canvas").getContext("2d");
    context.fillText("Hello, world!", 10, 10);
</script>