Drum plots with backtrace_time and x_scale not well choosen

[sbonaime]

If the user choose a backtrace_time which is not a divisible by x_scale, the plots seems to be not in real time. Maybe we should prevent the user to do so.

If backtrace_time % x_scale:
print 'backtrace_time should be a mutltiple of x_scale'
exit(0)