|Normalized Weekly Abundance
The periods of 1990-1999 and 2000-2009 were analysed and displayed separately to allow any shift in
flight time over the last two decades to be discerned. The tick marks on the week number axis are
between the two bars for that week's data. (This is how Google Charts does it).
The heights of the bars of the two periods have been normalized to show abundance
as if the same number of records had been submitted during each period.
This allows crude comparison of the species' abundance in both decades.