Using data in a model for plotting to chart

Bert59

Hi,

I have a list of data comming from a text file.
I load this file on the C++ side and have made a model to display these data in a QML ListView.

Now I would like to plot these data on a ChartView.
How can I access the data in the model from outside the ListView in order to add them to a LineSeries which can be displayed on a ChartView?

Thnaks

Bob64

@Bert59 since your data is provided by a C++ backend it might make sense to populate your line series in C++. You should be able to instantiate a LineSeries object in your QML and pass that into a C++ API function that you expose to QML (probably via a context property). The C++ function can receive the line series argument as a QAbstractSeries and use the C++ API of that class to populate the series data.

If I recall, the Oscilloscope example demonstrate how to do this. I think that was my starting point when I was figuring out QML charts. I'd recommend you take a look at it.

oria66

Hi, @Bert59.

You have a class or a function in the C++ side that receives a QAbstractSeries:

void YourClassExportedToQML::update(QAbstractSeries *series)
{
    if (series)
    {
        QXYSeries *xySeries = static_cast<QXYSeries *>(series);

       // Here you replace the data in the xySeries with your data
       // use replace, it's better for performance

    }
}

Now in the QML side, I supposed you have the class YourClassExportedToQML and it has a signal when the data came:

 Connections{
        target: yourClassExportedToQML
        onMySignalWhenDataIsReady: {

                var serie = chart.createSeries(ChartView.SeriesTypeLine,
                                               "MySerie",
                                               axisX,
                                               axisY);

                yourClassExportedToQML.update(serie);
            
        }
    }

If you have doubts, do not hesitate to ask!

Bert59

Thank you Bob64 and oria66 for your replies.

I must say that I'm a beginner with Qt and I find the oscilloscope example quite complicated to understand.
Would it be possible to send me a minimum project with a few hard coded values using your hints oria66?

Bert59

Hi oria66,
Just wanted to let you know that finally I succeeded to implement your code examples
Thank you

Bert59

Hi, just an additional question.
Using your suggestion I have added below
QXYSeries *xySeries = static_cast<QXYSeries *>(series);
the following:

QList<QAbstractAxis *> axis;
axis = xyseries->attachedAxes();
axis.at(0)->setMax(120);
This allow setting the max of the X axis.

Unfortunately I could not find how retrieve the actual setting. There is no getMax or something similar available.
Do you have an idea how to do that ?

GrecKo

What I would use instead to answer the original question is a VXYModelMapper

Bert59

Thank you for the suggestion.
Could you provide an small example on how this class has to be implemeted?
From the documentation it's not so obvious when you are a beginner with Qt.
Thanks

oria66

Hi @Bert59. When I worked with charts I remember that I manage the axis value from the QML side.
For example:

        DateTimeAxis {
            id: axisX
            min: new Date(2020,3,30,0,0,0)
            max: new Date(2020,3,30,24,0,0)
            format: "hh:mm"; labelsColor: "white"
        }

You can manipulate the min and max value from axisX.

However, it would be interesting in implementing the idea proposed by @GrecKo.