print-composerqgisqgis-3
In QGIS 3 there is the possibility to illustrate proportional data like population with circle of proportional size with the "assistant" option.
But once in the print composer section, there is an empty symbol in the legend.
How to show 2 or 3 circles of proportional size in the legend?
I don't use the proportional circles plugin bu can suggest an alternative method which will give you the control you need (and without creating another layer):
Edit from the Expression section of the popup menu and create a formula. To define a start size simple begin you formula with a numeric value (or field value). So for hectares you could have an expression something like: 10 + (area/10000).I suggest to :
Duplicate the layer (Right click on it in the layer panel and duplicate)
Give a small size to the duplicated layer
Refresh in the print composer ( you will have two "Coniferous Tree" Layers)
Uncheck the "Auto Update button", select the Bigger layer and remove it from the legend
Best Answer
I've found a working solution within QGIS3:
It works with a point layer
Then go back to the general "Marker" option on the Symbology tree and at the bottom go to the "advanced" button option then "Data defined size legend".
You will be able to make a nice legend with proportionnal circle symbol