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.   Small Skipper   Essex Skipper   Large Skipper   Dingy Skipper   Grizzled Skipper   Clouded Yellow   Brimstone   Large White   Small White   Green-veined White   Orange Tip   Green Hairstreak   Purple Hairstreak   White-letter Hairstreak   Black Hairstreak   Small Copper   Small Blue   Brown Argus   Common Blue   Chalkhill Blue   Holly Blue   Duke of Burgundy   White Admiral   Purple Emperor   Red Admiral   Painted Lady   Small Tortoiseshell   Peacock   Comma   Dark Green Fritillary   Silver-washed Fritillary   Speckled Wood   Wall Brown   Marbled White   Gatekeeper   Meadow Brown   Ringlet   Small Heath