in rspec_stats/rspec_query.rb [138:148]
def pct_history(days, rows)
last = nil
days.map do |day|
next_row = rows.find { |r| r['day'] == day }
pct = next_row && last ? diff_percent(last, next_row) : 'n/a'
last = next_row
pct
end.drop(1).join(', ')
end