How to label points with gradient-colored data-defined rectangles in QGIS 3.18?

Question

I am trying to symbolize a point layer with rectangles shaped and colored by gradient according to attribute values. I am trying to do this via label backgrounds rendered as "Geometry generator" marker symbols to prevent rectangles from overlapping and enable easy callout creation (and to enable gradient fill -- this is, to my knowledge, impossible with Ellipse markers).

Here is an overview of what I am trying to achieve. The white rectangles (these are label background symbols) are placed as I want, but I cannot color them properly. The colored rectangles are created with geometry generator, they are the desired symbols, but I cannot place them correctly.

I encountered a feature of geometry generator I cannot explain. Let us take the following table as an example point layer:

x   y
10  12
15  27
43  43
27  15
12  10

When I pass the code $geometry to geometry generator in the Symbology section, the points are placed where they are meant to be:

However, when I add labels and pass the same code to geometry generator in Labels > Background > Marker symbol, the points are placed well away from the point locations:

These labels change their locations as I drag the map canvas, they enclose the point layers when the points are near the upper left corner of canvas. The locations of text labels remains as usual.

I have two (three) questions:

1. Why are the geometry-generated label background symbols placed like that?

2. How can one avoid such improper placement?

(3.) Maybe there is another way to symbolize points with geometry generator and prevent them from overlapping that is less painful?

Answer 1

For what you want to do, it is probably better to use Filled Marker or Points displacement style renderer instead of Single Symbol.

Screenshot: Filled marker with label:

Screenshot: compare the two rendering options. Points displacement (right side) allows you to define a Distance and Placement method:

If you really want to use label backgrounds, be sure to have proper setting in the placement tab: I used Offset from Point, and placed it in the middle.

Screenshot: red dots (original point symbol) and blue rectangle (label Background):

Answer 2

I did not manage to make label backgrounds work properly. I made a clumsy workaround using point symbology and the Expression Builder since I am not experienced with PyQGIS classes and functions.

This is the original data with attributes I wanted to depict with rectangles:

pop x   y   min max n   placing_x   placing_y
11  -2387981.505    3775859.307 1.01    1.13    4   right   down
12  -1941488.19 4181736.9   0.99    1.13    1   right   down
13  53559.889   2291172.327 1.01    1.12    4   left    up
14  50652.236   2291661.88  1.02    1.1 1   left    down
7   -3016462.464    4607316.088 0.98    1.1 1   left    up
8   -2968065.604    4431503.4   0.98    1.06    1   right   down
9   -3002923.802    4454940.99  0.99    1.09    1   right   up
10  -2092972.683    3321772.21  0.99    1.14    4   right   down
S2  -5801526.999    4622477.218 1.05    1.27    5   right   down
S3  -4576538.657    4279330.501 1.22    1.3 5   right   down
S4  -4586348.397    4283898.882 1.18    1.3 5   right   up
S5  -4637773.282    4278981.752 1.17    1.23    5   left    down
15  -1964593.017    4269980.772 1.01    1.11    1   right   up
21  -3165463.862    3909144.758 0.93    1.05    3   left    down
24  -3307648.848    4099644.049 0.98    1.1 2   left    down
S1  -5801526.999    4622477.218 1.15    1.41    5   right   up
S6  -4956384.622    4171352.355 1.27    1.33    5   left    up
S7  -3895551.651    2375986.041 1.21    1.33    5   right   up

I wanted rectangles to be as long as (max - min) and as high as n. The respecitive gradient coloring should have represented the part of the overall gradient ramp ranging from the minimum min value to the maximum max value. I created variables placing_x and placing_y for the convenient adjustment of rectangle position and filled them manually.

The resulting expression for the rectangle follows:

with_variable('x_offset', ("max" - "min") * 0.1 * @map_scale,
with_variable('y_offset', "n" * 0.0005 * @map_scale,
with_variable( 'rect', make_rectangle_3points(
make_point($x - @x_offset, $y - @y_offset), 
make_point($x - @x_offset, $y + @y_offset),
make_point($x + @x_offset, $y + @y_offset)
),
CASE
WHEN "placing_y" = 'up' AND "placing_x" = 'right' THEN
translate(@rect, 0.005 * @map_scale, 0.004 * @map_scale)
WHEN "placing_y" = 'down' AND "placing_x" = 'right' THEN
translate(@rect, 0.005 * @map_scale, -0.004 * @map_scale)
WHEN "placing_y" = 'up' AND "placing_x" = 'left' THEN
translate(@rect, -0.005 * @map_scale, 0.004 * @map_scale)
WHEN "placing_y" = 'down' AND "placing_x" = 'left' THEN
translate(@rect, -0.005 * @map_scale, -0.004 * @map_scale)
END
)))

The data-defined gradient color ramp boundaries are set as follows:

(aggregate(@layer, 'min', "min")-"min") / ("max" - "min")

for the left boundary and

1 + (aggregate(@layer, 'max', "max")-"max") / ("max"-"min")

for the right boundary.

Finally, I added the text labels using pop field with Cartographic positioning and data-defined position priority:

CASE
WHEN "placing_y" = 'up' AND "placing_x" = 'right' THEN 'TR'
WHEN "placing_y" = 'down' AND "placing_x" = 'right' THEN 'BR'
WHEN "placing_y" = 'up' AND "placing_x" = 'left' THEN 'TL'
WHEN "placing_y" = 'down' AND "placing_x" = 'left' THEN 'BL'
END

Ta-da!