Each square is 1% of the electricity generated, coloured by source.
chart.type: 'waffle' is an alias for the unit chart's
grid layout with square cells;
plotOptions.unit.grid.total: 100 rounds the values (largest
remainder) to exactly 100 cells, so the grid always reads as percentages.
Switching scenario is a plain chart.updateSeries(); the grid
keeps every cell in its slot, so only the cells at each category boundary
recolour. The unit / waffle chart is a premium type; without a license it
renders with a trial watermark.