title: drawing over an Image in HTML5 Canvas while preserving the image
body: <p>In HTML5 Canvas, what's the simplest way to draw <em>and move</em> a line over an Image (already on the canvas), preserving the image underneath?  (e.g. have a vertical line track the mouse X position)</p>
  <p>My current canvas:</p>
  <pre><code>$(document).ready(function() {
    canvas = document.getElementById(&quot;myCanvas&quot;);
    context = canvas.getContext(&quot;2d&quot;);
    imageObj = new Image();
  
      imageObj.onload = function() { 
      context.drawImage(imageObj, 0,0);  
    }
    imageObj.src = &quot;http://example.com/some_image.png&quot;;
    $('#myCanvas').click(doSomething);
  });
  </code></pre>