This map shows the distribution of all types of Plough Monday custom, not only folk plays, but also plough trailing, house visiting, molly dances, sword dances, etc., that took place on Plough Monday. This has been redrawn from a map originally presented in a paper on the origins of Plough Monday at the Traditional Drama '79 conference and updated in 1980 (Millington, 1979). This is what the paper says about the map
"Map 1 illustrates the distribution of all known forms of Plough Monday custom. It has been plotted using a technique used extensively in biological mapping, in which the whole of a given l0km National Grid square is coloured in if there is at least one recorded occurrence in that square. This method both speeds the plotting of the map and makes general distribution patterns easier to see.
Only records which name a specific location could be could be plotted on the map, or to put it another way, it has not been possible to plot records which refer generally to a county. It is interesting to note that I have not been able to find records of specific occurrences in several counties for which there are general references (e.g. Northumberland), and it seems probable that these references may just be extrapolations from the customs of neighbouring counties. In the case of Northumberland however, it may be that some of the sources use the name in the literal sense of "land north of the Humber". However this would be unusual usage.
To a certain extent, the distribution reflects collecting patterns. There are concentrations in the areas where plays and dances have been extensively collected, i.e. Grid squares SK and TL, and the area just south of the Humber. Conversely, the distribution is sparser where non-play and non-dance customs seem to have predominated, e.g. Yorkshire and Northamptonshire."
You can compare this map with regional variations in the traditional times of appearance of folk plays and the actors' collective names.
The Origins of Plough Monday,
Traditional Drama '79, One Day Conference, University of Sheffield, 20th Oct.1979