To change data source name you need to check its current value and then issue rrdtool command.

You can check data source (DS) name using commands:

$ rrdtool info graph_memory.rrd  | grep ^ds | sed 's/.*\[\(.*\)\].*/\1/g' | uniq
memFree

$ rrdtool dump graph_memory.rrd | grep name
  memFree 

To change data source name from memFree to memoryFree issue command:

$ rrdtool tune graph_memory.rrd -r memFree:memoryFree