code: function timeInterval(pxPerSec) {
      var retval = 1;
      if (pxPerSec >= 25 * 100) {
          retval = 0.01;
      } else if (pxPerSec >= 25 * 40) {
          retval = 0.025;
      } else if (pxPerSec >= 25 * 10) {
          retval = 0.1;
      } else if (pxPerSec >= 25 * 4) {
          retval = 0.25;
      } else if (pxPerSec >= 25) {
          retval = 1;
      } else if (pxPerSec * 5 >= 25) {
          retval = 5;
      } else if (pxPerSec * 15 >= 25) {
          retval = 15;
      } else {
          retval = Math.ceil(0.5 / pxPerSec) * 60;
      }
      return retval;
  }
docs: Use timeInterval to set the period between notches, in seconds,
  adding notches as the number of pixels per second increases.
  
  Note that if you override the default function, you'll almost
  certainly want to override formatTimeCallback, primaryLabelInterval
  and/or secondaryLabelInterval so they all work together.
  
  @param: pxPerSec