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I am new to QGIS and can't figure out how to display a legend for the dot size of a centroid, simple marker. The size of the dot is dependent on a value from a CSV linked to a layer map of Europe. I have two dots per country (different years). This is a snippet of the map:

enter image description here

Now what I need is to be able to display a legend for the size of the dots. It is showing me a legend in the "symbol size" menu: enter image description here

I was able to create a legend while using graduated symbols. I also tried using graduated and then switch method to "size". However, even in a new project, the only option I get is color:

enter image description here

So this is not an option either. I'm using QGIS 3.8.

Edit: Conerning Josephs answer: I can't find this optione. I do have the advanced dropdown on certain levels, but not on all of them. Here with Screenshots: First level: enter image description here Second level: enter image description here Third level: enter image description here Fourth level: enter image description here

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Once you have defined the field for your Size parameter, there is an advanced option where you could set the data-defined size legend if you click on the top main marker. Here you can set the size legend manually or use the values automatically from the field you specified:

Data-defined size legend

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  • Hey Jospeh, Thanks for your help! However, I can't find this option. I did edit the first post and add some screenshots. It also seems that I have three levels for the marker and you only two (Marker -> Simple marker). For me it is centroid Fill -> Marker -> Simple Marker. Might this be the reason why I can't find the option? Thanks & best, Stefan
    – stibi
    Commented Jul 2, 2019 at 19:20
  • @Stefan - Apologies, when you mentioned using a csv I assumed you were using a point shapefile which is where this option is available. I don't think it's possible using it for a line/polygon shapefile. An alternative could be to create a new point layer which uses data from the csv or polygon layer and then add the size legend.
    – Joseph
    Commented Jul 4, 2019 at 10:11
  • Hey Jospeh, no need to apologize - you're trying to help :) I figured that ou myself by now. My idea was using the vector polygons tools -> Polygon centroids. This way I get a point layer. The only problem: with the linked csv I was able to uncheck "Draw point on part of every multi-part features". I can't do that now and this means that there X points per country. Any input here? I also can't filter the attibute tables becaus it does not include any coordinates.
    – stibi
    Commented Jul 4, 2019 at 18:27
  • @Stefan - You could run the Dissolve tool and select the country field. This merges all features which have the same country value.
    – Joseph
    Commented Jul 5, 2019 at 9:16

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