title: canvas fillStyle in interval with transparent/opacity
body: <p>This small code clear old canvas data in interval:</p>
  <pre><code>// start interval
  ctx.save();
      ctx.fillStyle = &quot;rgba(0, 0, 0, 0.2)&quot;;
      ctx.fillRect(0, 0, ctx.width, ctx.height);
  ctx.restore();
  //some draw code for new graph
  ...
  //end interval
  </code></pre>
  <p>My work area become black, because I set black as fill color (<code>rgba(0, 0, 0, .2)</code>), but I need a transparent background, not black.</p>
  <p>I tried use <code>globalAlpha</code> and <code>imagePutData</code> but I failed.</p>
  <p>How I can do this?</p>