As I already knew, most of my followers are based in the UK, but it was interesting to see how many are from the US, and mainly concentrated along the east coast. And as I (hopefully!) gain more followers with time, mapping them should be more and more useful.
Because not all Twitter users include a geographical location in their bio (which is the information used by the application), the map doesn't show everyone, and only one of my numerous Cardiff-based followers appears.
There were also a couple of anomalies. One Cardiff-based research centre is shown as being in Wales, South Africa, and a London-based research centre is shown as being in Germany! I suppose the Google-powered map uses the information it finds to the best of its ability. But it did make me realise that it's quite helpful to include several locations (e.g. town + county) when you complete this section of your Twitter bio.