Fill between is for filling between two y-values over a range of
x-values. Is there anything which fills between to x-values over a
range of y-values?
Nothing with the ease of use of fill_between, but you can always write your own PolyCollection, which is what fill_between does (see the function implementation for details) or create a Polygon for a simple region. My use cases are typically in the time series world where I have datetime on the x-axis and some range of values on the y. If folks think it is sufficiently useful to have a fill_betweenx function with a similar interface, you could probably fairly easy port fill_between to fill_betweenx.