Election night pendulum?

Longman Dictionary of Contemporary English Word definitions in Longman Dictionary of Contemporary English
noun EXAMPLES FROM CORPUS ▪ A Radio Times election supplement shows the new swingometer . ▪ But in such a setting Peter Snow's neon swingometer was, frankly, a disappointment.

Wikipedia Word definitions in Wikipedia
The swingometer is a graphics device that shows the effects of the swing from one party to another on British election results programmes. It is used to estimate the number of seats that will be won by different parties, given a particular national swing ...

Wiktionary Word definitions in Wiktionary
n. (context British English) a graphic device used to illustrate swing in an election