The y span of the geometry is set by the equation, however the y span of the Custom object itself sets an upper limit for the y span of the geometry. If the y value of the equatin is greater than the y span of the Custom object the equation curve will be cut off. For example:n

In your case you can set the y span to be a large value. As long as it is larger than the maximum y span of the taper it should work.n