I suppose digital vectorscopes don't have to bother with that old
ntsc weirdness, though I thought maybe they'd keep it for consistency (shrug),
with the zero axis being at the far left, representing the "vomit yellow" of
colorburst, and then rotating clockwise through the color circle towards
yellow, red, magenta, etc. This way a grayscale (zero chroma) ended up
as a straight horizontal line going to the left.

    Oops, I'm wrong about that last bit with a grayscale; on a vectorscope a grayscale
    would just be a dot at the center, lol.

    It was colorburst I was thinking of that marched to the left on a vector scope,
    and they had a special target for it so you wouldn't mistake it for part of the visible image.

    Since all NTSC signals had colorburst in them, even a completely black image,
    it would be seen at all times on a vectorscope as a short line from the center
    off to the left (red circle below).

    Colorburst was normally never seen as a color since it was hidden in horizontal blanking,
    but could be made visible in the handy "cross pulse" mode of most studio monitors:

   

    Burst was an unmistakeable mustard/vomit yellow bar in the blanking area.