empty graph?

The rrd in question has been built by mrtg (although I'm not going to be using that going forward - I just need to get the process of using externally populated rrds right). I can point 14all or routers2.cgi at the rrd file and it shows me the data I know is in there, but cacti shows a blank graph.

I've set the DS name as both ds0 and ds1 (although can anyone tell when which one is used for what?), but still no output. Below is the graph cmd (please ignore my dumb paths - I'm just doing some test setups & was feeling lazy with apache!);

E:/rrdtool/src/tool_release/rrdtool.exe graph - \ --imgformat=PNG \ --start="-604800" \ --title="jaws_host_cpu.rrd" \ --base=1000 \ --height=120 \ --width=500 \ --alt-autoscale-max \ --vertical-label="" \ DEF:a="/inetpub/mrtg/htdocs/cacti/rra/jaws-w2k-mibii_host_cpu.rrd":ds1:AVERAGE \ LINE1:a#:""

The start of an rrdtool info looks like; rrd_version = "0001" step = 300 last_update = 1046892891 ds[ds0].type = "GAUGE" ds[ds0].minimal_heartbeat = 600 ds[ds0].min = 0.0000000000e+000 ds[ds0].max = 1.0000000000e+002 ds[ds0].last_ds = "UNKN" ds[ds0].value = 1.7460000000e+003 ds[ds0].unknown_sec = 0 ds[ds1].type = "GAUGE" ds[ds1].minimal_heartbeat = 600 ds[ds1].min = 0.0000000000e+000 ds[ds1].max = 1.0000000000e+002 ds[ds1].last_ds = "UNKN" ds[ds1].value = 1.7460000000e+003 ds[ds1].unknown_sec = 0

Any pointers much appreciated.

ta