The source code is here r-tmap/tmap.cartogram#1, but I'd like to add this to the core tmap functionality.

But the question is: where and how should the user specify a transition, and how is it processed internally?

Using the tmap.cartogram package, this is the single cartogram:

library(tmap)
library(tmap.cartogram)

Africa = World[World$continent == "Africa", ]

tm_shape(Africa, crs = "+proj=eck4") +
tm_cartogram(size = "pop_est")

Where?

• As a new map variable called "frame", which can either be a data variable, or a number indicating the number of frames.
• In the tm_opt_x options list.

How?

• Number of frame should be specified
• The transition mode, linear, cubic-in-out, and (what are the other options?)

Internals:

• Use it as a facet-dimension. Just like tmap animations are usually made
• Alternatively, we can see it as a separate thing, and extend the geometry: instead of one feature per tmapID, we could have a list of features, one for each time stamp. This is probably much harder to implement.