Creating a waterfall plot with the spectrogram

[stilgar]

Sounds like something I did recently. In my case re-rendering the entire spectrogram was too slow. So I added a caching layer to get the speed I needed.

[..]

If you need it pixel perfect you might have to fiddle with it a bit. I think it still had an off by one problem in the cached image.

Hi xilcoy,
I've a couple of questions about your class Waterfall and in particular about renderImage method.
First of all i d'dn't understand why the third parameter is const QRectF; shouldn't it be const QwtDoubleRect instead?
Then, the other thing that is not clear to me is if the area must be different each time or if it's always the same. In this case, in fact, xShift and yShift are 0. If area should change, how can you change it?

Thx