Some examples and a few words at http://www.thinkbroadband.com/faq/sections/bqm.html#313
On yours specifically, fairly normal for Virgin Media.About half the ones I quickly looked at have similar yellow sized area for Virgin Media
Seeing red and the blue getting wider is a sign of bad problems.
my graph, wide blue bit while downloading some MP3's from Amazon, and other spikes are generally web browsing. Red spike was a power cut we had during the night.