Is there a statistic in garmin connect to see the platform center offset (pco) over multiple rides? It would be interesting to see the overall tendency!
WKO4 (commercial / $ closed source) desktop app, Golden Cheetah (free / open-source) desktop app or TrainingPeaks (on-line free or $) application can perform analyses on your raw cycling data (or running etc.). I use WKO4, which is highly customizable and will let you see your data over arbitrary time frames. There are gobs of pre-defined charts and tables freely available from their on-line library and you can define your own if you wish (and contribute them to the library, if you like).