Our SChartAnnotations are just a subclass of UIView, so you can create any annotation you like including the one in your image.
For more information on our annotations, see our Annotations sample in your Shinobi download.
One thing I would say, is it looks a lot like our SChartSeriesCrosshair we've built into the chart.
To see our series crosshair in action assign one to your chart like so:
chart.crosshair = [SChartSeriesCrosshair new];
Then enable the crosshair on your series like so:
series.crosshairEnabled = YES;