An exception occurred when I used SplinePoint and Spline to generate a curve. I wanted to generate a curve according to the 4 points and 4 vectors in the figure below. The curve between the second point and the third point should be a straight line segment, but the curve generated by SplinePoint and Spline is instead a curve. 

How do I get the curve I want? Is there a bug? The same operation on Grasshopper results in the following figure.