Allow custom event timeseries in stats API

“We are building a rudimentary ad service for a website where we already use Plausible for traffic analytics. The clicks on the ads are tracked as custom events, and the (numeric) identifier of the ad is added as a custom property to the event.

This way we are hoping to be able to answer questions using Plausible’s stats API like “how many times was a given ad clicked in total”, or “what was the distribution of clicks for each day since the ad is live”. The latter is currently not possible, hence this change proposal.”