given a time, return the corresponding x value for paper
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;
  }