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In QGIS 3.10.2 I have a map that I want to show points by colour using one category and size by another. I have tried to follow the instructions here: https://docs.qgis.org/3.10/en/docs/user_manual/working_with_vector/vector_properties.html#proportional-symbol-and-multivariate-analysis. This I have done but a consequence is that in the Layout composer the legend symbol is either missing or very tiny.

This is my map with just a colour classification:

Just a colour classification

I then add the instructions to re-size by a second attribute (I converted a 4 category variable into the numbers 1, 2, 3 and 4 and called it SizePoint)

To add a second classification by size

Now the legend in the Layout composer does not show a dot at all

Missing legend dot

Now here this isn't a problem because all the dots are of the same INSTANCE. But there are other maps where there are more than one instance:

Missing two legend dots

You will see that there is no indication in the legend what INSTANCE 1 or INSTANCE 2 are.

I did try and look at the very specific help text here:

enter image description here

But I do not know what the classification frame is or see a Change button.

Any suggestions?

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1 Answer 1

Reset to default
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  1. In QGIS 3.16.0 I did as described in the docs (in my case I applied a graduated rendering of the value "Tabell_Endring_Areal"):

    enter image description here

  2. Click on the Symbol-button to the change it (I'm using circle) and then on the Data defined override-button and choose Assistant:

    enter image description here

  3. Choose source (the value (field) you want its symbol to be proportional. This is another value than the "graduated one" used in step 1), start and end value and output. NB. Set a minimum size for NULL:

    enter image description here

  4. Go back to the Layer styling-window and click on Advanced and Data-defined Size Legend:

    enter image description here

  5. Choose Separated or collapsed legend. The appearance in the Layers panel will change accordingly.

    enter image description here

Now the symbols also will be visible in the Layout:

enter image description here

So there is no bug, but it may not seem too intuitive to have to set a size when null. @gioman revealed this in a comment at https://github.com/qgis/QGIS/issues/39882

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