I'm using showArea: true but can't find the appropriate setting to make that fill all the way. Example:

I assume this is because I don't have any data points after where it ends, but I don't want the green line to extend all the way across the top. Is there another way to accomplish this?
You're using showArea:true to render the area.  But as you've noted, showArea fills the area associated only with the drawn line.
What you're looking for is an additional area without a line.
In order to achieve this effect, you'll need to create two different lines:  The first line will have showArea: false and extend to W3 as shown in your example.  This will render the light green line as you have already.
The second line will be invisible have showLine: false and showArea: true and extend all the way across the top to W8.
In other words, you're actually looking to render two different things. One is a line, and one is a fill area.
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