Calcuate Consumer lag from JMX beans in Kafka 0.7.2

[Xuyen On]

I would like to know what these attributes mean in my Kafka 0.7.2 brokers. I 
see the following in JConsole for my eventdata topic:

kafka.logs.eventdata-0
  Attributes
      Name                                          // Name of partition
      Size                                               //Is this the current 
number of messages?
      NumberofSegments             // Don't know what this is 
      CurrentOffset                           //Is this the current offset of a 
consumer?
      NumAppendedMessages   // Don't know what this is

Can someone please tell me what these attributes mean? I am trying to find out 
if I can use these values to calculate the consumer lag ie: Size - 
CurrentOffset and report it to a chart or dashboard somewhere for real-time 
analysis.