Sets the value of StartDirection attribute. StartDirection is the direction of the tangent at the start point. Direction value 0 indicates a curve with a start tangent along the positive x-axis. Values increases counter-clockwise, and decreases clockwise. Depending on the plane angle unit, either degree or radians, the sensible range is -360° ≤ n ≤ 360° (or -2π ≤ n ≤ 2π). Values larger than a full circle (>|360°| or >|2 π| are not used.