i am trying to do a Pie charts in my ruby on rails app

i am using the gem gruff in window 7 with docker but when i try to write the g.data doesn t allow me for this error:

Magick::ImageMagickError in ArticlesController#index
unable to read font `(null)' @ error/annotate.c/RenderFreetype/1362
Extracted source (around line #32):


puts "mira para que revises"
puts g
g.write("pie_keynote.png")




Rails.root: /example
Application Trace | Framework Trace | Full Trace

app/controllers/articles_controller.rb:32:in `index'

and this is my code of articles_controller line 32:

g = Gruff::Pie.new
g.title = "Visual Pie Graph Test"
g.data 'Fries', 20
g.data 'Hamburgers', 50
puts "mira para que revises"
puts g
g.write("pie_keynote.png")

and i really don t get why the error :(