Currently Stock chart does not have "init" handler. We will add it in upcoming versions.

Meanwhile you can use "zoomed" event. It's called when the chart is first initialized so you can use first call to "zoomed" handler to catch when chart is fully initialized.

var inited = false;
chart.addListener('zoomed', function (event) {
  if (!inited) {
    // remove "loading" panel
    // ...
    inited = true;
  }
});