Bug in Network Traffic example

# 🐛Bug report

**Describe the bug**
Hovering over the Data In or Data Out div will cause the tracker to flicker.

**To Reproduce**
Steps to reproduce the behavior:
1. Go to http://software.es.net/react-timeseries-charts/#/example/traffic
2. Hover on Data In or Data Out
3. Observe the date time and tracker flicker

**Expected behavior**
Mouse should be able to hover over the div

**Desktop:**
Chrome 73 and Firefox 66

**Additional context**
handleMouseOut fires causing the tracker to return null.