Rare event charts are used to plot intervals between events. Time between events tends to follow a Weibull distribution. The T chart is used to track whether the intervals between events are changing or remain stable.

If we were to plot the number of events in a day or week, it may be possible to use this data as a c-chart that displays a count of events. The problem with this technique is that rare events tend to have a rate that is very close to zero. Any event that occurs will be flagged up as unusually high.

Here we use the T chart to plot the time between eruptions of Vesuvius. Vesuvius is historically a very active and dangerous volcano, which was responsible for the destruction of Pompeii and Herculaneum in 79 AD. The dataset that we are using tracks the start dates of the eruptions from the year 1000 onwards.