Animating horizontally ordered symbols

Many Isotype charts contain horizontal rows of symbols. An example is the classic population pyramid. An Isotype version can be seen below:

A common practice is to animate a single pyramid over time. Applying this to the Isotype population pyramid means that we have to introduce a user interface element to select a year and that the selected year defines the number of displayed symbols.

The question is: How do the symbols behave during the transition of one year to another year?

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