Is it possible to tell cacti to read last value if a 0 value is returned.

Usage graph from an ISP reads off a perl script but is unreliable site so always looks like this

You would need to use a script to do that because the default SNMP options assume that 0 is valid. I don't thin there's an option to override that (could be a future enhancement request if opened on GitHub). Your script would basically see the 0 and return a U instead.

SpikeKill has a gap filling feature. I would recommend you increase the heartbeat on your template and your RRDfiles to something larger. That way the LAST good item will stay good for longer. If you wanted the graph to not gap for say like a day without getting data, set the heartbeat to 86400.