    curve.tangentAtT(t)

Return a line that is tangent to the curve at point specified by `t`.

The function expects `t` to lie between 0 and 1; values outside the range are constrained to 0 and 1, respectively.

The tangent line starts at the specified point. The direction from `start` to `end` is the same as the direction of the curve at the specified point.

If the control points of the curve all lie at the same coordinates, `null` is returned (it is impossible to determine the slope of a point). The `curve.isDifferentiable()` function may be used in advance to determine whether tangents can exist for a given curve.