sounds as if it could be done with QV ;)
Make a cyclic group with the kinds of periods you want to see (months, weeks, quarters...).
Summarize over time, make a second expression with the period minus 1 (with set-expression).
Whereas period stands for week, month or whatever you have chosen.
And then you can divide, for example with column(2)/column(3). So you get your value bigger or smaller than 1 :)
I tried your tip but it seems that I don't fully understand where to insert the "period - 1" statement... Do I add it to the cyclic group when I creat it (under Settings -> Document properties) or in the chart that I've added? I just can't find this set -> expression button that you described...
And another small question - you said that I can devide the results of the two periods to see if it's bigger than one or not. Can I creat a Gauge chart and connect that result to it? I mean: 1 will be the center line, above it will be the right area and below it will be the red area...