In my version of ubuntu (12.04)
bootchart creates a different log file every time the computer boots, this goes into the directory
bootstartgui looks by default to the file
/var/log/bootchart.tgz which does not exist.
In 12.04, if you install
bootchart properly, you automatically get an image file of the boot graph representation, so you do not need to run
In fact, it is enough to run (Alt+F2):
then double click on the image that contains the date and time that you are interested in.
If you want to delete useless logs and save space, you can start nautilus with sudo from the commandline: Type (Alt+Ctrl+T) and then:
gksu nautilus /var/log/bootchart
This will give you a window where you can also delete files. Be very cautious not to delete anything on another folder!