Not too sure that I'm interpreting the diagram correclty, I'm just checking my assumptions:

1) What's the significance of circles? Do they represent voting districts or something like that?

2) What does the size of a circle represent? Is it the population of a district, perhaps on logarithmic scale?

3) Does the position on the x-axis represent how strongly that district was for remain or leave?

Of course, you could give the link to the original source too, Google Image Search shows nothing yet.