I took 15 minutes, but mine is 4 lines

homework.rb

require 'rubygems'
require 'open-uri'
require 'json'

url = 'http://twitter.com/statuses/user_timeline.json?id=visnup&count=3200'
dates = open(url) { |f| JSON.parse f.read }.map { |e| Date.parse e['created_at'] }
sum = dates.inject(Hash.new(0)) { |s, t| s[t] += 1; s }

sum.keys.sort.each { |k| puts "#{k} (#{sum[k]}) #{'=' * sum[k]}" }