If there are "too many" data sets the generated graph has a clipped legend line. That makes it impossible
to know what some of the data sets represent (since you don't get the small color box)
Using the demo as an example, the legend is clipped at Peaches and Apples3

#!/usr/bin/env ruby

require 'rubygems'
require 'gruff'

g = Gruff::Line.new
g.title = "My Graph" 

g.data("Apples", [1, 2, 3, 4, 4, 3])
g.data("Oranges", [4, 8, 7, 9, 8, 9])
g.data("Watermelon", [2, 3, 1, 5, 6, 8])
g.data("Peaches", [9, 9, 10, 8, 7, 9])
g.data("Apples2", [1, 2, 3, 4, 4, 3])
g.data("Oranges2", [4, 8, 7, 9, 8, 9])
g.data("Watermelon2", [2, 3, 1, 5, 6, 8])
g.data("Peaches2", [9, 9, 10, 8, 7, 9])
g.data("Apples3", [1, 2, 3, 4, 4, 3])
g.data("Oranges3", [4, 8, 7, 9, 8, 9])
g.data("Watermelon3", [2, 3, 1, 5, 6, 8])
g.data("Peaches3", [9, 9, 10, 8, 7, 9])

g.labels = {0 => '2003', 2 => '2004', 4 => '2005'}

g.write('my_fruity_graph.png')