The additional time axis appears when you are viewing the chart with a Historical date range rather than in Realtime. It allows you quickly narrow down what the chart is showing you to specific points of interest by clicking and dragging on the axis. While it only shows one tag at a time, other selected tags can be viewed by right clicking on the axis and selecting a different tag.
Here is a screen recording that demonstrates some of the behavior:
Currently there isn’t a way to disable this functionality. If it is something that you would want, I can put it in as a feature request.
Thanks,
Garth