Geometry Generator - Straight lines not curved

Question

I have QGIS 3.8.2 installed and started to look at the Geometry Generator. I have a layer of airline tracks and trying to create a curved line linking start and end points.

I did a lot of searching and found the followng code but when I apply this I get straight lines not curved lines like I have seen elsewhere.

make_line(
    start_point($geometry),
    centroid(
        offset_curve(
            $geometry, 
            length($geometry)/-5.0
        )
    ),
    end_point($geometry)
)

Hopefully its something simple that I am doing wrong.

Answer 1

To visualize them as curved lines, change the Symbol layer type to Arrow, then select Curved arrows option.

Head length/thickness should be zero as you would not need arrow-heads.