Shading a line chart above a set threshold only

[5880forre]

Hopefully someone can assist, YouTube has not been as helpful as usual!

I am attempting to format a line graph with shading.

I need to track a daily temperature reading in degrees and I would like any instances over a set threshold (26 degrees for instance) to be shaded.

All the YouTube tutorials I have seen only seems to let me do the shading above and below the line graph. I only need to highlight areas above the threshold, as such shading below the threshold just complicates the graph.

Any help would be greatly appreciated!

[#2]

@5880forre , Does this work?

[#3]

@5880forre Please view the attached chart. Currently I manually set both primary and secondary axis's lower bound to 20 and upper bound to 30 to enlarge the shading area. You can edit them to whatever bound you want but need to change both axis with  the same scale.