Calcuate Consumer lag from JMX beans in Kafka 0.7.2

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.