QGIS » QGIS Application

The aim is to obtain the Multivariate example screenshot at https://docs.qgis.org/testing/en/docs/user_manual/working_with_vector/vector_properties.html#proportional-symbol-and-multivariate-analysis and as far as i can say, when the data defined is applied to the symbol size, categories symbols are vanished. They are still applied in the map canvas but not shown in layer properties --> Symbology page, neither in layers panel nor in the layout legend.

If not clear, step by step (and i think i've narrowed the source of the issue while reporting):
- Apply a categorized symbology on a point layer
- Then, click on the Change... button to configure the default symbol of the categories
- While At the marker level, use the data defined widget next to the size property:

• if you select an existing field or enter an expression, it works well:you get size scaling and categories are shown
• if you use the assistant entry, things work well if and only if (afaics) "size when null" is different from 0. In that case, scaling is done but categories symbols are cleared. Actually it turns out that the category symbol is based on the null size, given that the symbol is updated in the layer tree as you play with the value applied to null features. This does not make sense to me as null values are particular one you may want to treat differently (not show them) and it should instead, in case we really need to have sized symbols for categories, use the overridden value of the property (in this case, the grayed value of size near the DD button)