Is it possible to have a background under the spark line?

[littlepea]

Like this:

[mfreeman451]

this is an area chart, not a line chart

[jonalxh]

Is it working? I've tried with fill-opacity option and nothing happened.

[jonalxh]

I've tried to set fill property due to in the docs you say whatever property not supported will be asumed by SVG, it puts the property fine, but is added to svg tag and not to path tag. How to change it?