Creating horizontal lines connecting points using QGIS Geometry generator

Question

I am using the geometry generator to learn how to generate vectors using expressions. Currently, I have a point vector, and I would like to create horizontal lines (shown in red: manually drawn) connecting the points, as illustrated in the image below:

Requires Lines

I am aware that the make_line function allows us to create lines. How should I modify my code to generate a line that passes through these points horizontally?

The condition I intend to use is to connect lines with the same Y coordinates.

make_line(collect($geometry, group_by:=y($geometry))

Answer 1

Easiest is probably

make_line (array_agg ($geometry, group_by:=$y))

Or, extending your version, you can add a geometries_to_array() function: it is available since QGIS 3.28, so for older versions use the expression above:

make_line( 
 geometries_to_array (
 collect ($geometry, group_by:=$y)
 )
)

enter image description here

• unknown123: Is there any way to extract this generated line in any vector format? or is this only useful for visualization purpose?
• unknown123: I tried using 'Geometry by expression' but there is error 'Evaluation error: Cannot use aggregate function in this context.'
• Babel: Yes, Geoemtry by expression would be the way to do so. Unfortunately, as you realized, it does not work with aggregate functions. Maybe post this as a separate question.