    curve.tangentAt(ratio [, opt])

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

The function expects `ratio` 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.

The function uses the same algorithm as the `curve.tangentAtLength()` function.

The default value for `opt.precision` is 3; this corresponds to maximum observed error of 0.1%.

The `opt.subdivisions` property may be specified, directly providing an array of pre-computed curve subdivisions from which to calculate curve length. Use the `curve.getSubdivisions()` function to obtain an array of curve subdivisions. The `opt.precision` property is still necessary, however; it determines the precision of the point search algorithm.