I recently ran into a problem where Cacti was not graphing 10 GigE links on a Cisco 6509 Chassis. A friend of mine recommended a workaround of creating a 64bit graph and then switching the graph to a 95th Percentile after the fact. No matter what I did the system just did not want to properly collect the information. Thanks to the Cacti Forums I was able to resolve the problem by manually altering the rrd file to allow for more data.
First you want to check the settings for the rrd file:

# rrdtool info file.rrd
ds[trafficin].max = 1.0000000000e+08
ds[trafficin].lastds = "649239500934920"
ds[trafficout].max = 1.0000000000e+08
ds[trafficout].lastds = "1154119467695233"

Notice that the lastds values are larger than what the max supports. I increased this to unlimited with the following command:

# rrdtool tune file.rrd -a trafficin:U

rrdtool tune file.rrd -a traffic_out:U

And the system is graphing exactly as it should.
I need to look into the code and change the default rrd creation from 32bit to 64bit – is there really any reason why you would want it the other way?