order_legend_flip.py

# See https://gis.stackexchange.com/questions/385525/qgis-how-to-invert-the-values-in-a-layout-legend

def invertLegendOrder():
    layer = iface.activeLayer()
    view_model = iface.layerTreeView().layerTreeModel()
    toc_layer = view_model.rootGroup().findLayer(layer)
    cat_count = view_model.legendRootRowCount(toc_layer)
    order_property = toc_layer.customProperty('legend/node-order')
    # print(order_property)

    if ( order_property is not None ) & ( order_property != "empty" ): # A custom node order property has already been set
        current_order = [int(i) for i in order_property.split(',')]
        new_order = [i for i in reversed(current_order)]
    else:
        current_order = [i for i in range(cat_count)]
        new_order = [i for i in reversed(current_order)]

    QgsMapLayerLegendUtils.setLegendNodeOrder(toc_layer, new_order)
    view_model.refreshLayerLegend(toc_layer)
    # order_property = toc_layer.customProperty('legend/node-order')
    # print(order_property)

def removeCustomLegendOrder():
    layer = iface.activeLayer()
    view_model = iface.layerTreeView().layerTreeModel()
    toc_layer = view_model.rootGroup().findLayer(layer)
    toc_layer.removeCustomProperty('legend/node-order')
    view_model.refreshLayerLegend(toc_layer)