some_dbi_stuff.rb

require 'json'
require 'time'

data = select_from_db.to_a

out = {
  :item => data.map{|key, value| value.inspect },
  :axisx => [
    data.first.first,
    data[data.size / 2].first,
    data.last.first,
  ].map{|datetime| Time.parse(datetime.to_s).strftime('%a %H:%M') },
  :axisy => [data.values.min, data.values.max],
  :colour => 'ff9900',
}

puts out.to_json