Bug report #19099
Data defined Size legend is wrong when using separated symbols
|Affected QGIS version:||3.4.1||Regression?:||Yes|
|Operating System:||Easy fix?:||No|
|Pull Request or Patch supplied:||No||Resolution:|
|Crashes QGIS or corrupts data:||No||Copied to github as #:||26929|
The data-defined size symbols feature has a set of unhandled issues i reported months ago (see list) but i think this one is a really broken feature that can lead to wrong information on maps.
Apply a scaled size to a point layer
At the bottom of the properties, choose to enable the data defined Size Legend
In the new dialog, choose "separated legend items", do the other settings and apply.
Most of the times, the symbols won't scale in the preview frame nor in the layers panel but the size displayed in the layers panel, hence in the layout legend, is really wrong. Really bigger than what's displayed in the map canvas.
If you switch to the "collapsed size symbols", you'll notice the difference of size and that sizes are coherent now.
#2 Updated by Alexandre Neto about 2 years ago
I have also noticed this problem. From some testing, seems that the symbols size in the legend is using the actual values on the data, and not the scaled values provided by the expression, for example.
coalesce(scale_exp("capacity", 0, 2717, 1, 10, 0.57), 0).