QGIS 3 - Wrong legend for datadefined size points layer

Question

I am trying to generate a proportional symbol symbology using data defined symbol size in a QGIS 3 python plugin.

Here is the working code :

# Symbol creation
symbolLayer = QgsSimpleMarkerSymbolLayer()
symbolLayer.setShape(QgsSimpleMarkerSymbolLayer.Circle)
symbolLayer.setSize(2.0)
symbolLayer.setColor(QColor(31, 120, 180)
symbolLayer.setStrokeColor(QColor(255, 255, 255)
symbolLayer.setStrokeWidth(0.4)

symbol = QgsMarkerSymbol.createSimple({})
symbol.changeSymbolLayer(0, symbolLayer)

# Size definition (Flannery method)
# layer is a QgsSingleSymbolRenderer
strExp = 'coalesce(scale_exp("value", 0.0, 150, 1, 20, 0.57), 0)'
symbol.setDataDefinedSize(QgsProperty.fromExpression(strExp))
layer.renderer().setSymbol(symbol)

The problem is the legend. This code :

# Legend
lgd = QgsDataDefinedSizeLegend()
lgd.setLegendType(QgsDataDefinedSizeLegend.LegendCollapsed)
lgd.setClasses([
    QgsDataDefinedSizeLegend.SizeClass(150, '150'),
    QgsDataDefinedSizeLegend.SizeClass(75, '75'),
    QgsDataDefinedSizeLegend.SizeClass(15, '15')
])

layer.renderer().setDataDefinedSizeLegend(lgd)

produces an enormous legend symbol :

However, if I open the layer properties dialog and update parameters with datadefined size assistant, the legend is refreshed with the correct size :

What am I missing ?

Answer 1

first of all as you are working on QGIS 3, the way you define the size is not wrong but now there's a class which allows to do this directly:

objTransf = QgsSizeScaleTransformer(
        QgsSizeScaleTransformer.Flannery,
        0,
        150,
        0,
        150,
        0,
        0.57)

you'll find more about this class here:

QgsSizeScaleTransformer

you have to create a QgsProperty, set its Field to strFieldName, set its transformer to objTrans and send it as the dataDefinedSize of your symbol.

objProp = QgsProperty()
objProp.setField(strFieldname)
objProp.setTransformer(objTransf)
symbol.setDataDefinedSize(objProp)