I believe it is a shorthand for 'whites who were present before the creation of the USA' which has been conflated with the English speaking lower class immigrants and those from other groups who have been assimilated by marriage, linguistic replacement and cultural identification into that group. The geographical distribution of this 'demographic' is very much what I have seen elsewhere identified as Scots-Irish, redneck, etc. The definitions for the graph are that the color represents the group with the largest number of members per county or parish.

"It is not necessary to have hope in order to persevere."