21.2.5. The Legend Item
The Legend item is a box or a table that explains the meanings of the symbols used on the map. A legend is then bound to a map item. You can add a legend item with the Add Legend tool following items creation instructions and manipulate it the same way as exposed in Interacting with layout items.
By default, the legend item displays all available layers and can be refined using its Item Properties panel. Other than the items common properties, this feature has the following functionalities (see 图 21.26):
The Main properties group of the legend Item Properties panel provides the following functionalities (see 图 21.27):
In Main properties you can:
Change the Title of the legend. It can be made dynamic using the data-defined override setting, useful for example when generating an atlas;
Choose which Map item the current legend will refer to. By default, the map over which the legend item is drawn is picked. If none, then it falls back to the reference map.
Variables of the linked map item (@map_id, @map_scale, @map_extent...) are also accessible from data-defined properties of the legend.
Wrap the text of the legend on a given character: each time the character appears, it's replaced with a line break;
Set the symbols and text placement in the legend: the Arrangement can be Symbols on left or Symbols on right. The default value depends on the locale in use (right-to-left based or not).
Use Resize to fit contents to control whether or not a legend should be automatically resized to fit its contents. If unchecked, then the legend will never resize and instead just stick to whatever size the user has set. Any content which doesn't fit the size is cropped out.
The Legend items group of the legend Item Properties panel provides the following functionalities (see 图 21.28):
The legend will be updated automatically if Auto update is checked. When Auto update is unchecked this will give you more control over the legend items. All the icons below the legend items list will be activated.
The legend items window lists all legend items and allows you to change item order, group layers, remove and restore items in the list, edit layer names and symbology and add a filter.
The button uses expressions to customize each symbol label of the selected layer (see Data-define the legend labels)
The Filter legend by expression helps you filter which of the legend items of a layer will be displayed, i.e. using a layer that has different legend items (e.g., from a rule-based or categorized symbology), you can specify a boolean expression to remove from the legend tree, styles that have no feature satisfying a condition. Note that the features are nevertheless kept and shown in the layout map item.
While the default behavior of the legend item is to mimic the Layers panel tree, displaying the same groups, layers and classes of symbology, right-click any item offers you options to hide layer's name or raise it as a group or subgroup. In case you have made some changes to a layer, you can revert them by choosing Reset to defaults from the contextual menu of the legend entry.
After changing the symbology in the QGIS main window, you can click on Update All to adapt the changes in the legend element of the print layout.
allows you to add expressions to
each symbol label of a given layer. New variables (
@symbol_count) help you interact with the legend entry.
For example, given a
regions layer categorized by its
you can append to each class in the legend their number of features and total area,
Borough (3) - 850ha:
Select the layer entry in the legend tree
Enter the following expression (assuming symbol labels have not been edited):
format( '%1 (%2) - %3ha', @symbol_label, @symbol_count, round( aggregate(@layer, 'sum', $area, filter:= "type"=@symbol_label)/10000 ) )
For all item types it allows to modify the label text by typing in or by inserting expressions using the Insert or Edit an Expression. Expressions can also be added directly anywhere in the item's label by using the [% expression %] notation.
The Legend Item Property also allows you to control the column splitting behaviour by forcing the column split to occur after a specific item or all symbols of a layer. Automatic splitting of a layer and its child can also be allowed or blocked on a layer-basis in this widget.
For items with a symbol, the Legend Item Property allows you to specify the maximum height and width that a symbol can occupy.
For vector symbols, you can specify a custom shape for the symbol. The shapes are usually defined by an expression to represent the geometry in a simple plane, but those symbols can also be saved in the style manager and imported later. The default symbol for each geometry type can also be controlled via the style manager.
A custom symbol can also be specified for vector symbols. This can be useful to tweak the render of a specific symbol, to enhance it in the legend or have a symbol independent from its true symbol preview. This custom symbol will override the legend symbol, but will take into account the symbol Patch specified.
The Fonts group of the legend Item Properties panel provides the following functionalities:
You can change the font of the legend title, group, subgroup and item (feature) in the legend item using the font selector widget
For each of these levels you can set the text Alignment: it can be Left (default for left-to-right based locales), Center or Right (default for right-to-left based locales).
You set the Color of the labels using the color selector widget. The selected color will apply to all the font items in the legend.
Under the Columns group of the legend Item Properties panel, legend items can be arranged over several columns:
The Symbol group of the legend Item Properties panel configures the size of symbols displayed next to the legend labels. You can:
The WMS LegendGraphic section of the legend Item Properties panel provide the following functionalities (see 图 21.32):
When you have added a WMS layer and you insert a legend item, a request will be sent to the WMS server to provide a WMS legend. This Legend will only be shown if the WMS server provides the GetLegendGraphic capability. The WMS legend content will be provided as a raster image.
WMS LegendGraphic is used to be able to adjust the Legend width and the Legend height of the WMS legend raster image.
The Spacing section allows you to customize the spacing within the legend. Spacing can greatly help denote the groupement of items in the legend and their relation.
Spacing around and before title, groups, subgroups, symbols, labels, boxes, columns and lines can be customized through this dialog.