I’ve developed a filter to give me a YUV vectorscope to help me when I am developing filters. I thought it would be useful for other people too so I have added it to Testing -> Iain Fergusson in the plugin.
I also added a bunch more options. You can plot any channel against any other channel (Y,U,V,L,a,b,c,h,C,M,Y,K,R,G,B) and view it a few different ways. One thing I quite like is that you can plot colour with the circle size scaled inversely proportionally to luminance, so that 1) dark colours are easier to see, and 2) you can see a range of luma values for pixels that are at the same x/y coordinates because the lighter colours circles do not entirely cover the darker colours.