You should be able to compare graphs as long as they arrre done on the same dyno. All the newer types such as the dynojet 200/250 use softwear to compensate for air humidity,air pressure and temperature. If you ran your bike on a wet day and then again 6 months later on a sunny day. If the bike had not be altered the reading should be less than about 1bhp differance.
All Dyno jet softwear takes readings 3 times a second to insure that runs can be compaired to later runs. If for some reason the softwear feels that the condition differances are too great for an comparison a warning is displayed.
The idea that you can't compaire dyno readings from the morning to the afternoon comes from a time when the dyno was a "real" brake and before every run a person had to take readings such as air pressure and temp. Of course some people were lazy and didnt, meaning that the graphs that were calculated meant bugger all as all the factors had changed in the time between runs. Its now all done by the computer so is pretty much error free.
Of course there are still ways aroudn things... If one run is done with the dyno room pressurised and then the next without the increase or decrease in power will be put down to engine performance not conditons. This is where you have to find a trusted dyno where everything is done of a level playing feild.
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If I have to take the carbs off once more...