    line.tangentAt(t)

Return a line tangent to the line at point that lies `t` (normalized length) away from the beginning of the line.

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 line.

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