Thanks for getting in touch!
The InterpolatePoints api you are referring to is located on the SChartCrosshair class.
The ShinobiChart's Crosshair property is of type ISChartCrosshair, which is a protocol that both SChartCrosshair and SChartSeriesCrosshair implement.
To turn off InterpolatePoints you can create a new SChartCrosshair object and set the property to false like so:
SChartCrosshair ch = new SChartCrosshair(chart);
ch.InterpolatePoints = false;
chart.Crosshair = ch;
Let me know if you have any questions.