Issue with scruffy

Hi im trying to render a simple graph for the moment just to test
the library but i'm having a problem... i try to run this example
from other page.

graph = Scruffy::Graph.new
    graph.title = "Favourite Snacks"
    graph.renderer = Scruffy::Renderers::Pie.new

    graph.add :pie, '', {
      'Apple' => 20,
      'Banana' => 100,
      'Orange' => 70,
      'Taco' => 30
    }

    graph.render :to => "pie_test.svg"
    graph.render :width => 300, :height => 200,
      :to => "pie_test.png", :as => 'png'

and i get an error in
graph.render :width => 300, :height => 200,
:to => "pie_test.png", :as => 'png'

no such file to load -- RMagick

i haven't found in any place something that says that scruffy needs
rmagick to work... it's really necesary to install it for render the
graph in a view?

I only ask because i'm having problems to install rmagick, it says that
it failed building the native extencion even if i have installed the
devkit