d3.interpolateObject(a, b)
Returns an interpolator between the two objects a and b. Internally, an object template is created that has the same properties as b. For each property in b, if there exists a corresponding property in a, a generic interpolator is created for the two elements using interpolate. If there is no such property, the static value from b is used in the template. Then, for the given parameter t, the template's embedded interpolators are evaluated and the updated object temp