See http://bl.ocks.org/monfera/10527804 for the motivation. This example uses transitions with, or among projections.
It is preferred that a transformation is carried out by changing the parameters of a projection (translate, rotation, parallels etc.). For example, the small multiple line chart, the spiral chart and the transition between them is using the same conic projection, with edge cases of secant parallels and their tweening. Note: cartographic projections are less than ideal for regular charts, but d3.js has a well developed set of geo functions, enabling rapid experiments in line with the motivation linked above - moreover, cartographic projections are themselves necessary for visualizing the often present geographic dimensions.
If it is not possible to stay within one projection, then we need to interpolate between two projections that have an identical edge case, to preserve a continuous visual flow.
It is therefore possible to have a smooth transi