This was prompted by parents in Kent saying they didn't know children in other parts of England don't have to sit a test to apply for a secondary school place. 
There is a brief write up here 
https://trak.org.uk/which-local-authority-has-most-children-in-grammar-schools-kent-or-buckinghamshire

Data was downloaded from DfE. 
https://explore-education-statistics.service.gov.uk/find-statistics/school-pupils-and-their-characteristics
It needs manually extracting from the zip file. This repository includes the latest data at time of its creation. 
(The script selects the latest year's data.) 

The plot margins need manually adjusting to get it on the page.