last time a state voted for the respective party. darkest = 2012, lightest = 1964/1972, respectively.
This sequence could not have started before 1976, because Mississippi, Alabama, South Carolina and Texas, which voted Democratic last time in 1976 have the same color than Oklahoma, Kansas, Nebraska, Utah, Idaho and Wyoming, which voted Democratic last time in 1964. If the sequence started in 1964, Mississipi, Alabama, South Carolina and Texas would be darker than Oklahoma, Kansas, Nebraska, Utah, Idaho and Wyoming.
I think it means last time a state voted for the respective party since 1980. Darkest = 2012, lightest = no vote
