title: get touchstart coordinate on current element
body: <p>So I have an element that looks like this:</p>
  
  <pre><code> ____________________________________
  /                                    \
  |                                    |
  |                                    |
  +------------------------------------+
  |                                    |
  |                                    |
  |                                    |
  +------------------------------------+
  |                                    |
  |                                    |
  \____________________________________/
  </code></pre>
  
  <p>I have attached a touchstart listener to it, like this:</p>
  
  <pre><code>    other_options.addEventListener('touchstart', function(e) {
          event.preventDefault();
      }, false);
  </code></pre>
  
  <p>What I want to do, and I've already looked through the values of 'e' but I can't find any value consistent enough (values don't seem right to me when I try them) to do what I want.</p>
  
  <p>I know the size of those rows. I just want to be able to take the Y coordinate of where the touchstart event fired, being 0 the upper coordinate of the element. That way, Math.floor(y / ROW_SIZE) will give me the row the touchstart event was started on.</p>