NoMethodError: undefined method on basic example

[WilyWildWilly]

On an iRuby notebook (IRuby 0.4.0, Ruby 2.7.0, Ubuntu 18.04.1), while following the first tutorial on Nyaplot diagrams, as I try to use a DataFrame i get a NoMethodError: undefined method `<' for nil:NilClass

The code I try to run:

plot2 = Nyaplot::Plot.new
plot2.add_with_df(df, :bar, :species, :number) # x-> column :species, y-> column :number
plot2.show

The error I get (full trace):

NoMethodError: undefined method <' for nil:NilClass /home/xxx/.rbenv/versions/2.7.0/lib/ruby/gems/2.7.0/gems/nyaplot-0.1.6/lib/nyaplot/diagram.rb:70:in process_data'
/home/xxx/.rbenv/versions/2.7.0/lib/ruby/gems/2.7.0/gems/nyaplot-0.1.6/lib/nyaplot/diagram.rb:16:in initialize' /home/xxx/.rbenv/versions/2.7.0/lib/ruby/gems/2.7.0/gems/nyaplot-0.1.6/lib/nyaplot/plot.rb:75:in new'
/home/xxx/.rbenv/versions/2.7.0/lib/ruby/gems/2.7.0/gems/nyaplot-0.1.6/lib/nyaplot/plot.rb:75:in `add_with_df'

:1:in `' /home/xxx/.rbenv/versions/2.7.0/lib/ruby/gems/2.7.0/gems/iruby-0.4.0/lib/iruby/backend.rb:44:in `eval' /home/xxx/.rbenv/versions/2.7.0/lib/ruby/gems/2.7.0/gems/iruby-0.4.0/lib/iruby/backend.rb:44:in `eval' /home/xxx/.rbenv/versions/2.7.0/lib/ruby/gems/2.7.0/gems/iruby-0.4.0/lib/iruby/backend.rb:12:in `eval' /home/xxx/.rbenv/versions/2.7.0/lib/ruby/gems/2.7.0/gems/iruby-0.4.0/lib/iruby/kernel.rb:90:in `execute_request' /home/xxx/.rbenv/versions/2.7.0/lib/ruby/gems/2.7.0/gems/iruby-0.4.0/lib/iruby/kernel.rb:49:in `dispatch' /home/xxx/.rbenv/versions/2.7.0/lib/ruby/gems/2.7.0/gems/iruby-0.4.0/lib/iruby/kernel.rb:38:in `run' /home/xxx/.rbenv/versions/2.7.0/lib/ruby/gems/2.7.0/gems/iruby-0.4.0/lib/iruby/command.rb:110:in `run_kernel' /home/xxx/.rbenv/versions/2.7.0/lib/ruby/gems/2.7.0/gems/iruby-0.4.0/lib/iruby/command.rb:40:in `run' /home/xxx/.rbenv/versions/2.7.0/lib/ruby/gems/2.7.0/gems/iruby-0.4.0/bin/iruby:5:in `' /home/xxx/.rbenv/versions/2.7.0/bin/iruby:23:in `load' /home/xxx/.rbenv/versions/2.7.0/bin/iruby:23:in `'