code: function(ratio, opt) {
  
              if (!this.isDifferentiable()) return null;
  
              if (ratio < 0) ratio = 0;
              else if (ratio > 1) ratio = 1;
  
              var t = this.tAt(ratio, opt);
  
              return this.tangentAtT(t);
          }
docs: Returns a tangent line at requested `ratio` with precision better than requested `opt.precision`; or using `opt.subdivisions` provided.