Patches for Linux kernel (v4l code) and for v4l-utils should be applied to obtain I&Q constellation described in this article.
Patch for Linux kernel: http://stand.netup.tv/downloads/iq_constellation.patch
Patch for v4l-utils: http://stand.netup.tv/downloads/iq_constellation_v4l-utils.patch
Receiving I&Q data from DVB demodulator
dvb_const -f /dev/dvb/adapter0/frontend1 -o iq_dvbc_qam256.txt -p 100000
received data (100000 points) will be saved into iq_dvbc_qam256.txt file. File format is simple text csv. Example of received data:
-175,-45 -46,110 -241,-178 176,11 -50,173 16,44
Values are signed. First I part, second Q part.
Visualization of received data
dvb_const_plot utility build PNG image with constellation:
dvb_const_plot -i iq_dvbc_qam256.txt -o iq_dvbc_qam256.png -c "SNR=38dB QAM256"
 Constellation diagram
 Digital Transmission: Carrier-to-Noise, Signal-to-Noise & Modulation Error Ratio