Do not show mouse tracker if empty

[nicofrand]

In hit.js, line 312, the mouse tracker is filled and inserted even if the content returned by the trackFormatter function is empty (null, "", undefined, etc.).

mouseTrack.innerHTML = n.mouse.trackFormatter({
      x: x,
      y: y,
      series: n.series,
      index: n.index,
      nearest: n,
      fraction: n.fraction
    });
    D.show(mouseTrack);

[cesutherland]

I'll leave it if they return an empty string, but hide for null and undefined. Does that work for you?

[nicofrand]

That will be fine thanks !