- Qt 6.8
- Qt Graphs
- Simple Bar Graph
- Qt Graphs | Commercial or GPLv3

# Simple Bar Graph

Using Bars3D in a QML application.

Simple Bar Graph shows how to make a simple 3D bar graph using Bars3D and QML.

The following sections describe how to switch series and display more than one series at a time. For more information about basic QML application functionality, see Simple Scatter Graph.

#### 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.

#### Data

The example data set is the monthly income and expenses of a fictional company over several years. The data is defined in a list model in `Data.qml` like this:

     ListModel {
         id: dataModel
         ListElement{ timestamp: "2016-01"; expenses: "-4";  income: "5" }
         ListElement{ timestamp: "2016-02"; expenses: "-5";  income: "6" }
         ListElement{ timestamp: "2016-03"; expenses: "-7";  income: "4" }
         ...

Each data item has three roles: timestamp, income, and expenses. The timestamp value is in format: `<four digit year>-<two digit month>`. Usually, you would map years and months to rows and columns of a bar graph, but you can only show either income or expenses as the value.

Now, add the data to the Bars3D graph. Create two Bar3DSeries inside it, starting with a series for the income:

     Bar3DSeries {
         id: barSeries
         itemLabelFormat: "Income, @colLabel, @rowLabel: @valueLabel"
         baseGradient: barGradient

         ItemModelBarDataProxy {
             id: modelProxy
             itemModel: graphData.model
             rowRole: "timestamp"
             columnRole: "timestamp"
             valueRole: "income"
             rowRolePattern: /^(\d\d\d\d).*$/
             columnRolePattern: /^.*-(\d\d)$/
             rowRoleReplace: "\\1"
             columnRoleReplace: "\\1"
             multiMatchBehavior: ItemModelBarDataProxy.MultiMatchBehavior.Cumulative
         }
         ...

The data is attached to the `itemModel` property of the ItemModelBarDataProxy inside the series. For the `valueRole`, specify the `income` field as it contains the desired value. Getting the years and months is a bit more complicated since they are both found in the same field. To extract these values, specify the `timestamp` field for both `rowRole` and `columnRole`. Additionally, specify a search pattern and a replace rule for those roles to extract the correct portion of the field contents for each role. The search pattern is a normal JavaScript regular expression, and the replace rule specifies what the field content that matches the regular expression is replaced with. In this case, replace the entire field content with just the year or the month, which is the first captured substring for both rows and columns. For more information about the replace functionality with regular expressions, see the QString::replace(const QRegExp &rx, const QString &after) function documentation.

The `multiMatchBehavior` property specifies what to do in case multiple item model items match the same row/column combination. In this case, add their values together. This property has no effect when showing values for each month, as there are no duplicate months in our item model. However, it becomes relevant later when you want to show the yearly totals.

Then, add another series for the expenses:

     Bar3DSeries {
         id: secondarySeries
         visible: false
         itemLabelFormat: "Expenses, @colLabel, @rowLabel: -@valueLabel"
         baseGradient: secondaryGradient

         ItemModelBarDataProxy {
             id: secondaryProxy
             itemModel: graphData.model
             rowRole: "timestamp"
             columnRole: "timestamp"
             valueRole: "expenses"
             rowRolePattern: /^(\d\d\d\d).*$/
             columnRolePattern: /^.*-(\d\d)$/
             valueRolePattern: /-/
             rowRoleReplace: "\\1"
             columnRoleReplace: "\\1"
             multiMatchBehavior: ItemModelBarDataProxy.MultiMatchBehavior.Cumulative
         }
         ...

The model contains expenses as negative values, but they could be shown as positive bars for easy comparison to income bars. Use the `valueRolePattern` to remove the minus sign, and no replacement string needs to be specified as the default replacement is an empty string.

Use the `visible` property of the series to hide the second series for now.

#### Custom Axis Labels

`Axes.qml` redefines the category labels for the column axis because the data contains numbers for months, which would clutter the labels:

     Category3DAxis {
         id: columnAxis
         labels: ["January", "February", "March", "April", "May", "June",
             "July", "August", "September", "October", "November", "December"]
         labelAutoAngle: 30
     }

To make axis labels more readable at low camera angles, set automatic axis label rotation.

#### Switching Series

In `main.qml`, set up the graph and various UI elements. There are three interesting code blocks to highlight here. The first one shows how to change the visualized data between income, expenses, and both, by simply changing the visibility of the two series:

     onClicked: {
         if (text === "Show Expenses") {
             barSeries.visible = false
             secondarySeries.visible = true
             barGraph.valueAxis.labelFormat = "-%.2f M\u20AC"
             secondarySeries.itemLabelFormat = "Expenses, @colLabel, @rowLabel: @valueLabel"
             text = "Show Both"
         } else if (text === "Show Both") {
             barSeries.visible = true
             barGraph.valueAxis.labelFormat = "%.2f M\u20AC"
             secondarySeries.itemLabelFormat = "Expenses, @colLabel, @rowLabel: -@valueLabel"
             text = "Show Income"
         } else { // text === "Show Income"
             secondarySeries.visible = false
             text = "Show Expenses"
         }
     }

The axis label format and item selection label formats are tweaked to get the negative sign showing properly for expenses, which were actually resolved as positive values.

The second interesting block is where the visualized data is changed by adjusting the proxy properties:

     onClicked: {
         if (text === "Show yearly totals") {
             modelProxy.autoRowCategories = true
             secondaryProxy.autoRowCategories = true
             modelProxy.columnRolePattern = /^.*$/
             secondaryProxy.columnRolePattern = /^.*$/
             graphAxes.value.autoAdjustRange = true
             barGraph.columnAxis = graphAxes.total
             text = "Show all years"
         } else if (text === "Show all years") {
             modelProxy.autoRowCategories = true
             secondaryProxy.autoRowCategories = true
             modelProxy.columnRolePattern = /^.*-(\d\d)$/
             secondaryProxy.columnRolePattern = /^.*-(\d\d)$/
             graphAxes.value.min = 0
             graphAxes.value.max = 35
             barGraph.columnAxis = graphAxes.column
             text = "Show 2020 - 2022"
         } else { // text === "Show 2020 - 2022"
             // Explicitly defining row categories, since we do not want to show data for
             // all years in the model, just for the selected ones.
             modelProxy.autoRowCategories = false
             secondaryProxy.autoRowCategories = false
             modelProxy.rowCategories = ["2020", "2021", "2022"]
             secondaryProxy.rowCategories = ["2020", "2021", "2022"]
             text = "Show yearly totals"
         }
     }

To show the yearly totals, combine the twelve months of each year into a single bar. This is achieved by specifying a `columnRolePattern` that matches all model items. In this way, the series will only have a single column. The cumulative `multiMatchBehavior` specified earlier for the proxy becomes relevant now, causing the values of all twelve months of each year to be added up into a single bar.

To show just a subset of years, set `autoRowCategories` to false on the ItemModelBarDataProxy item and define the row categories explicitly. This way, only the items in the specified row categories are visualized.

The third interesting block shows how to get the row and column index of an item if you know the row and column values by using ItemModelBarDataProxy methods `rowCategoryIndex()` and `columnCategoryIndex()`:

     var timestamp = graphData.model.get(mainview.currentRow).timestamp
     var pattern = /(\d\d\d\d)-(\d\d)/
     var matches = pattern.exec(timestamp)
     var rowIndex = modelProxy.rowCategoryIndex(matches[1])
     var colIndex

     if (barGraph.columnAxis == graphAxes.total)
         colIndex = 0 // Just one column when showing yearly totals
     else
         colIndex = modelProxy.columnCategoryIndex(matches[2])

     if (selectedSeries.visible)
         mainview.selectedSeries.selectedBar = Qt.point(rowIndex, colIndex)
     else if (barSeries.visible)
         barSeries.selectedBar = Qt.point(rowIndex, colIndex)
     else
         secondarySeries.selectedBar = Qt.point(rowIndex, colIndex)

Example project @ code.qt.io