> Why is the date and time not saved in a usable format?
Well, it is.
But you are probably referring to a standardized format, where you can just click on it and it will produce a two-axis line graph – unfortunately such thing does not exist.

> I have great difficulties times quickly a log file as a graphic display.
If you have some experience with spreadsheet programs it shouldn’t take long to graph a single log file (took me around 5min here for my first).

However if you want to repeatedly graph log files, you are better of with a script in scilab or gnu octave. This way you can graph (and even generate PDFs) with a press of a button.
HTH