Yea it's unfortunate, and I wish there was a way to get it to show, but it hasn't bothered me enough to go and look at the scripts and try to figure it out.
From what I gather 6.0.x uses the .rrd files in /data/logger/db/data to generate the graphs, so you might be able to find a way to insert the old data into them... but I have no experience with working with rrd files so I'm just guessing here
