No citation needed.
Laughable :
I call complete BS on your assumption then, and claim it to be the other way around and only 1% are never plugged in - I'm clearly right and won't bother to citate this.
My friend has a Mitsubishi PHEV truck which has never been plugged in.
Well my sister always plugs her Outlander PHEV in, so on this very limited pool of data we're already at 50% from 99%!
They only have it for the tax benefits but do use it on battery mode in traffic like any other hybrid.
Well at least they're saving fuel and not giving people increased chances of lung cancer in traffic at least then