how to shade between two lines?

[Liu]

Hi.

I want to shade between two series, say s1 and s2. Observations of s1 is always bigger than s2 given the same time spot.
I wonder how can I shade the area between these two series. I've been trying to use area(l) s1 command, however, it doesn't allow me to
unshade the area below s2.

Many thanks if you have any idea on how to draw the shade.