I am using gruff to generate line charts in a rails project, putting together a quick proof of concept:

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class Chart
  def generate_line_graph
    g = Gruff::Line.new(400)
    g.data(:testing, [5, 0, 25, 10)
    g.write("tmp/line.png")
  end
end

I quickly ran into the following exception:

Magick::ImageMagickError: WriteBlob Failed `/Users/ME/projects/CLIENT/code/RAILS_APP/tmp/line.png' @ error/png.c/MagickPNGErrorHandler/1804
  from /Users/ME/.rvm/gems/ruby-2.1.0/gems/gruff-0.5.1/lib/gruff/base.rb:425:in `write'
  from /Users/ME/.rvm/gems/ruby-2.1.0/gems/gruff-0.5.1/lib/gruff/base.rb:425:in `write'
  from /Users/ME/projects/CLIENT/code/RAILS_APP/app/models/reports/chart.rb:36:in `generate_line_graph'
  from (irb):16
  from /Users/ME/.rvm/gems/ruby-2.1.0/gems/railties-3.2.16/lib/rails/commands/console.rb:47:in `start'
  from /Users/ME/.rvm/gems/ruby-2.1.0/gems/railties-3.2.16/lib/rails/commands/console.rb:8:in `start'
  from /Users/ME/.rvm/gems/ruby-2.1.0/gems/railties-3.2.16/lib/rails/commands.rb:41:in `'
  from script/rails:6:in `require'
  from script/rails:6:in `main'

The issue: the error is raised when writing to the tmp directory, changing to another directory and it worked as expected?

g.write("public/line.png") and g.write("public/tmp/line.png") both worked

Turns out this was a simple mistake on my part, I just provisioned a new machine and cloned the project repo – I did not have a tmp directory in my project yet, it would be nice if the error message could have just told me so.