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