`line.tangentAtLength(length)`

Return a line tangent to the line at point that lies `length`

away from the beginning of the line.

If negative `length`

is provided, the algorithm starts looking from the end of the line. If `length`

is higher than line length, a line tangent to the closest line endpoint is returned instead.

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.