Connect multipart polygon with lines to one label

Question

I have several one-part and multipart polygons that I want to label.

I Set X and Y values to move the label to a certain position and want to connect the label to the polygon using the following expression in a geometry generator:

make_line( point_on_surface( $geometry ), make_point( "Label_X" , "Label_Y" ))

I want the line to connect from the label to every part of a multipart Polygon.

Is this possible?

Answer 1

Fortunately, the centroid() function places a point at the centroid of each part of a multipart geometry. So you can use a simple expression to draw lines from the centroids to the label.

Add a geometry generator symbol layer to the polygon layer:

Use this expression:

make_line( centroid( $geometry ), make_point("Label_X" , "Label_Y"))

It looks like this (this is a multipart polygon with four parts):

Tweak the label offset settings to get the label directly over the point where the lines converge.

Answer 2

You can use the following code to create your center points for each part of your mutlipolygon:

centroid( geometry_n($geometry, @geometry_part_num ))

So your code for the lines will be:

make_line(centroid( geometry_n($geometry, @geometry_part_num )),make_point("Label_X" , "Label_Y"))

Replace centroid with point_on_surface if you want.

The result:

Tested on QGIS 3.4.2 and QGIS 2.18.7