Hi dalyto,
I wanted to provide you with a couple of resources that will help you moving forward. I've attached those below.
IMAQ Vision Concepts Manual:
http://www.ni.com/pdf/manuals/322916a.pdf
IMAQ Vision for LabVIEW User Manual:
http://www.ni.com/pdf/manuals/371007a.pdf
NI Vision 2014 for LabVIEW Help:
http://zone.ni.com/reference/en-XX/help/370281W-01/TOC24.htm
In the last link I belive what you're looking for would be the IMAQ Histograph or IMAQ Histogram. I hope this information helps!
Kale W.
Applications Engineer
National Instruments
support@ni.com