1) The darker the map gets, the earlier the contests are.
2) States vertically split in two on any of the maps are states that had contests on different dates for the two parties. Democratic contests are on the left and Republican contests are on the right.
This isn't anything new. These are the same maps that have been residing in the left sidebar (see said sidebar for the attendant primary calendars) for a while now, but the above is probably a better representation of the frontloading trend over time. Sure, you can scroll down the page or look at the slideshow of the changes, but this more immediately demonstrates the effect with each of the maps side by side.
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