require 'mapper'
#just initialize it with two known reference points on your map and you are ready to go!
mapper = Mapper.new({:lat=>48.13,:lon=>11.57,:x=>278,:y=>503}, {:lat=>53.55,:lon=>9.99,:x=>206,:y=>114})
#you can either add lat/lon and it will fill up the point with corresponding x/y
mapper.add_point(:lat=>52.52, :lon=>13.41)
#or add x/y and it will fill up the point with corresponding lat/lon
mapper.add_point(:x=>136, :y=>410)
#when you are done just work with the points (thinking about including a graphic gem for actual plotting)
mapper.each{|point| p point}
->  {:lat=>52.52, :lon=>13.41, :x=>361.848101265823, :y=>187.924354243542}
->  {:lat=>49.4257840616967, :lon=>8.45388888888889, :x=>136, :y=>410}