code: function time_x (t) {
      var self = this;
      if (t === null) {
        return null;
      }
      var delta = t - ui.capture_idx.start;
      if (delta < 0) {
        console.log('Negative delta for time ' + t);
      }
      var pix = delta * ui.pix_per_sec / 1000;
      var x = margin[3] + pix;
      return x;
    }
docs: given a time, return the corresponding x value for paper