- Qt 5.15
- Qt Data Visualization
- Qt Quick 2 Legend Example
- Qt Data Visualization | Commercial or GPLv3

# Qt Quick 2 Legend Example

Showing graph legend in a QML application.

The Qt Quick 2 legend example shows how to make an interactive legend for a graph.

The interesting thing about this example is displaying the legend. We'll concentrate on that and skip explaining the basic functionality - for more detailed QML example documentation, see Qt Quick 2 Scatter Example.

#### Running the Example

To run the example from Qt Creator, open the Welcome mode and select the example from Examples. For more information, visit Building and Running an Example.

#### Legend

The legend is simply a column of custom `LegendItem` items inside a transparent rectangle. Each item is supplied with a series and the graph theme:

     ColumnLayout {
         anchors.fill: parent
         anchors.margins: parent.border.width
         spacing: 0
         clip: true
         LegendItem {
             Layout.fillWidth: true
             Layout.fillHeight: true
             series: station1
             theme: barGraph.theme
         }
         LegendItem {
             Layout.fillWidth: true
             Layout.fillHeight: true
             series: station2
             theme: barGraph.theme
         }
         LegendItem {
             Layout.fillWidth: true
             Layout.fillHeight: true
             series: station3
             theme: barGraph.theme
         }
     }

The legend items consist of a marker rectangle, which indicates the color of the series, and a text field, which shows the name of the series. The colors we get from the series and the theme supplied at legend item initialization:

     property Theme3D theme
     property Bar3DSeries series
         ...
     RowLayout {
         anchors.fill: parent
         spacing: 0
         clip: true
         Item {
             id: markerSpace
             Layout.minimumWidth: 20
             Layout.minimumHeight: 20
             Layout.fillWidth: true
             Layout.fillHeight: true
             Layout.alignment: Qt.AlignVCenter
             Rectangle {
                 x: parent.x + parent.width / 4
                 y: parent.y + parent.height / 4
                 width: parent.width / 2
                 height: width
                 border.color: "black"
                 color: series.baseColor
             }
         }
         Item {
             height: markerSpace.height
             Layout.fillWidth: true
             Layout.fillHeight: true
             Layout.alignment: Qt.AlignVCenter
             Layout.minimumWidth: 100
             Text {
                 anchors.fill: parent
                 text: series.name
                 verticalAlignment: Text.AlignVCenter
                 clip: true
                 color: theme.labelTextColor
                 font: theme.font
             }
         }
     }

We want the legend to be interactive, so we add additional logic to enable selection of a series by clicking on a legend item, as well as highlighting the legend item corresponding to the selected series.

The highlight depends on the selection state of the series, so we define two states, which follow the Bar3DSeries::selectedBar property and adjust the `legendItem` color appropriately:

     states: [
         State  {
             name: "selected"
             when: series.selectedBar != series.invalidSelectionPosition
             PropertyChanges {
                 target: legendItem
                 color: series.singleHighlightColor
             }
         },
         State  {
             name: "unselected"
             when: series.selectedBar == series.invalidSelectionPosition
             PropertyChanges {
                 target: legendItem
                 color: theme.labelBackgroundColor
             }
         }
     ]

To make the legend item interactive, we define a MouseArea to detect clicks on it and adjust the series selection accordingly:

     MouseArea {
         id: mouseArea
         anchors.fill: legendItem
         onClicked: {
             if (legendItem.state === "selected") {
                 series.selectedBar = series.invalidSelectionPosition
             } else {
                 series.selectedBar = previousSelection
             }
         }
     }

The `previousSelection` used above is another custom property of `LegendItem`, which we update whenever selection changes on the series. This way we remember the last selected bar of each series:

     Connections {
         target: series
         onSelectedBarChanged: {
             if (position != series.invalidSelectionPosition) {
                 previousSelection = position
             }
         }
     }

Example project @ code.qt.io