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InfluxDB
Power Real-Time Data Analytics at Scale. Get real-time insights from all types of time series data with InfluxDB. Ingest, query, and analyze billions of data points in real-time with unbounded cardinality.
I think that most people likely do this outside of R (i.e. export their plot and add brackets in Illustrator). However, this looks like a straightforward package to add significance brackets and lets you specify exactly where to put them.
NOTE:
The number of mentions on this list indicates mentions on common posts plus user suggested alternatives.
Hence, a higher number means a more popular project.