Style sparklines
Add six years of data
Reduce height of sparklines
Make sparklines even less tall
Make first-time-buyer data span exactly 6 years
Remove log statement
Make minimum yScale value configurable for sparkline graphs
Add time period display to sparklines
Compare year-on-year figures, not month-on-month
Use tabular numbers in all styles
Rename common/single_stat to multi_stat_item, to avoid confusion with extensions/single_stat