I've been receiving a few queries about this recently. Seasonality does keep it's weather conditions data for an indefinite period of time, so the data older than 1 year is still available. The best way to view this data right now is to click and drag on the graph, to scroll back in time. However, this method of viewing past data isn't really ideal for comparing this year's weather to last year. I'm working on adding a method to show a side-by-side comparison of this year and last year, but it's hard to say when this will be ready.

Thanks for posting this feature request.