• Extensible types (XTypes)
• Abstract
• Motivation
• Summary
• Overview of adding new observation and aggregation types
• Calculating scalars
• Calculating series
• Calculating aggregates
• Registering your subclass
• Statically
• As a service
• Including in loop packets and archive records
• A comprehensive example
• The extension
• Registering the extension
• Performance
• Using the extension
• Cheetah
• Plot images
• Populate packets and records
• XTypes API
• Subtlety
• Other examples
• Alternatives to the chosen design
• Alternative: register functions with weewx.conf
• Alternative: register functions through the API
• Alternative: declare types
• Open issues