How to highlight a point on a simple line chart in NVD3?

I have a simple line chart in NVD3 like this http://nvd3.org/examples/line.html

I would like to highlight (overlapping a circle, for example) one specific point (given by x) in my series.

I have not found something similar to Highcharts addPoint () in NVD3, for example: http://api.highcharts.com/highcharts#Series

Moving the tooltip programmatically might work as well.

Thanks in advance!