`curve.tangentAtT(t)`

Return a line that is tangent to the curve at point specified by `t`

.

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 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.