Qt 5.13 | 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