Draw Rect with mixed xref


I want to gray out a plot past a certain point.
To do this, I want to draw a rectangle with a mixed xref where the first coordinate x0 refers to the left side of the viewport or 0 if xref is set to ‘paper’.
I’ve looked through some of the examples and documentation, and I know that if I set xref: ‘paper’ for the rect shape, I can draw x coordinates relative to the view port.

Is there a way to set the xref of x0 to paper while leaving xref of x1 to x (default), so I can draw a rectangle that covers everything from the left side of the viewport/plot up to some arbitrary point?
Or is there a way to convert an x coordinate on the plot to some value relative to the viewport/to the appropriate xref: ‘paper’ value or vice versa?

Not at the moment unfortunately.