Highlight parts of a 2D Line Chart in Numbers
Ah! Nevermind! I managed a workaround. Since I found no solution online I'll answer myself here in case it helps anyone.

Under Chart's Background &amp; Border Style, select "Advanced Gradient Fill" as shown in Image 1

Then change the Gradient angle to 180˚. If you hover just below the  Gradient Slider, you should see a + symbol. Use that to add more stops.

In the example below, I have 8 stops. (4 for each highlight zone). Each highlight zone would need 4 stops (in this order): 

White (Chart Background Color), Red (Highlight Color), Red (Highlight Color), and White (Chart Background Color). 


They are almost overlapping in pairs, and hence you can't see all 8 here. (Shown in Image 2)

The end result is shown in Image 3.

Sidenote: Had to club all 3 images into 1 image since you need at least 10 rep to post more than 2 links. (sigh!)