Hello Fabrice,
Currently the software limits the zero-span acquisition to ~4M points. We do not currently have plans to increase the number, but there is another way you could achieve a longer sweep time. There are record/playback control panels located along the bottom of the application in zero-span mode. You could set up a 30+ second recording and then play that back in the software. Playing back the recording is very convenient as you can scroll through the IQ data and control playback rates. Even with playback through, it will still only allow you to plot 4M points at once.
I hope this helps, let me know if you have follow up questions.
Regards,
A.J.