    path.tangentAt(ratio [, opt])

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

The function expects `ratio` to lie between 0 and 1; values outside the range are constrained to 0 and 1, respectively. If point at `ratio` is a point of discontinuity (e.g. it is a point shared by two Lineto segments with different slopes), the tangent line is constructed for the earlier segment (i.e. the segment closer to the beginning of the path).

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

The algorithm skips over segments that are not differentiable. This includes all invisible segments (e.g. Moveto segments) and visible segments with zero length. If the path contains no valid segments, `null` is returned. If the path has no segments at all, `null` is returned, as well. The `segment.isDifferentiable()` functions may be used to determine whether a given segment is valid; the `path.isDifferentiable()` function may be used to determine whether the path contains at least one valid segment.

The function uses the same algorithm as the `path.tangentAtLength()` function. It finds a valid segment which contains the point at length that corresponds to given `ratio` and then calls the segment's `segment.tangentAtLength()` function.

The `opt` argument is optional. Two properties may be specified, `opt.precision` and `opt.segmentSubdivisions`, which determine maximum error allowed in `pointAtLength` calculations for curved segments (default precision is 3; this corresponds to maximum observed error of 0.1%). The `opt.segmentSubdivisions` property is an array of individual segments' subdivision arrays. The `path.getSegmentSubdivisions()` function may be used to obtain the `segmentSubdivisions` array. The `opt.precision` property is still necessary, however; it determines the precision of the point search algorithm in curved segments.