just adding nyaplot gem (not even calling) gives me an error on osx (yosemite, 10.10.5)

[tansaku]

$ ruby lib/correlation.rb 
/Users/tansaku/.rvm/gems/ruby-2.2.3/gems/iruby-0.2.7/lib/iruby/utils.rb:8:in `display': undefined method `session' for nil:NilClass (NoMethodError)
    from /Users/tansaku/.rvm/gems/ruby-2.2.3/gems/nyaplot-0.1.6/lib/nyaplot/core.rb:45:in `init_iruby'
    from /Users/tansaku/.rvm/gems/ruby-2.2.3/gems/nyaplot-0.1.6/lib/nyaplot/core.rb:48:in `<module:Nyaplot>'
    from /Users/tansaku/.rvm/gems/ruby-2.2.3/gems/nyaplot-0.1.6/lib/nyaplot/core.rb:3:in `<top (required)>'
    from /Users/tansaku/.rvm/gems/ruby-2.2.3/gems/nyaplot-0.1.6/lib/nyaplot.rb:4:in `require_relative'
    from /Users/tansaku/.rvm/gems/ruby-2.2.3/gems/nyaplot-0.1.6/lib/nyaplot.rb:4:in `<top (required)>'
    from /Users/tansaku/.rvm/rubies/ruby-2.2.3/lib/ruby/2.2.0/rubygems/core_ext/kernel_require.rb:128:in `require'
    from /Users/tansaku/.rvm/rubies/ruby-2.2.3/lib/ruby/2.2.0/rubygems/core_ext/kernel_require.rb:128:in `rescue in require'
    from /Users/tansaku/.rvm/rubies/ruby-2.2.3/lib/ruby/2.2.0/rubygems/core_ext/kernel_require.rb:39:in `require'
    from /Users/tansaku/.rvm/gems/ruby-2.2.3/gems/daru-0.1.1/lib/daru.rb:45:in `block in create_has_library'
    from /Users/tansaku/.rvm/gems/ruby-2.2.3/gems/daru-0.1.1/lib/daru/plotting/vector.rb:39:in `<top (required)>'
    from /Users/tansaku/.rvm/rubies/ruby-2.2.3/lib/ruby/2.2.0/rubygems/core_ext/kernel_require.rb:54:in `require'
    from /Users/tansaku/.rvm/rubies/ruby-2.2.3/lib/ruby/2.2.0/rubygems/core_ext/kernel_require.rb:54:in `require'
    from /Users/tansaku/.rvm/gems/ruby-2.2.3/gems/daru-0.1.1/lib/daru/vector.rb:5:in `<top (required)>'
    from /Users/tansaku/.rvm/rubies/ruby-2.2.3/lib/ruby/2.2.0/rubygems/core_ext/kernel_require.rb:54:in `require'
    from /Users/tansaku/.rvm/rubies/ruby-2.2.3/lib/ruby/2.2.0/rubygems/core_ext/kernel_require.rb:54:in `require'
    from /Users/tansaku/.rvm/gems/ruby-2.2.3/gems/daru-0.1.1/lib/daru.rb:70:in `<top (required)>'
    from /Users/tansaku/.rvm/rubies/ruby-2.2.3/lib/ruby/2.2.0/rubygems/core_ext/kernel_require.rb:54:in `require'
    from /Users/tansaku/.rvm/rubies/ruby-2.2.3/lib/ruby/2.2.0/rubygems/core_ext/kernel_require.rb:54:in `require'
    from /Users/tansaku/.rvm/gems/ruby-2.2.3/gems/statsample-2.0.1/lib/statsample.rb:25:in `<top (required)>'
    from /Users/tansaku/.rvm/rubies/ruby-2.2.3/lib/ruby/2.2.0/rubygems/core_ext/kernel_require.rb:128:in `require'
    from /Users/tansaku/.rvm/rubies/ruby-2.2.3/lib/ruby/2.2.0/rubygems/core_ext/kernel_require.rb:128:in `rescue in require'
    from /Users/tansaku/.rvm/rubies/ruby-2.2.3/lib/ruby/2.2.0/rubygems/core_ext/kernel_require.rb:39:in `require'
    from lib/correlation.rb:2:in `<main>'

first two lines of the file:

``ruby require 'iruby' require 'statsample'

if I remove the nyaplot gem everything runs fine :-(