Using Scruffy - problem with point_markers

I’m trying to use the example showed in the scruffy page:

graph = Scruffy::Graph.new
graph.title = “Comparative Agent Performance”
graph.value_formatter =
Scruffy::Formatters::Percentage.new(:precision => 0)
graph.add :stacked do |stacked|
stacked.add :bar, ‘Jack’, [30, 60, 49, 29, 100, 120]
stacked.add :bar, ‘Jill’, [120, 240, 0, 100, 140, 20]
stacked.add :bar, ‘Hill’, [10, 10, 90, 20, 40, 10]
end
graph.point_markers = [‘Jan’, ‘Feb’, ‘Mar’, ‘Apr’, ‘May’, ‘Jun’]
graph.render :to => “stacking_test.svg”
graph.render :width => 500, :to => “stacking_test.png”, :as =>
‘png’

But I get the error:

C:/Ruby192/lib/ruby/gems/1.9.1/gems/scruffy-0.3.0.beta1/lib/scruffy/components/data_markers.rb:21:in
block in draw': undefined methodfirst’ for “Jan”:String
(NoMethodError)

… related with the line
graph.point_markers = [‘Jan’, ‘Feb’, ‘Mar’, ‘Apr’, ‘May’, ‘Jun’]

Does anybody know what happen and how to solve it?

Thanks in advance.