QML Chart View

[Rich Bair]

Is there a maximum number of XY series data points a QML Chart View can hold? Is it merely memory dependent or are there other limitations?

Thanks,
-Rich

[JonB]

@Rich-Bair Memory/efficiency, not a numeric limit (other than like max int).

[Beemaneni Bala 0]

@JonB
When i plot 3x line series with each around 30k points, the main thread slows down the processing. Below is the code snipped i used. After 40k it linearly decays the main thread functional behaviour i.e., the other timers / events does not fire in time

property var lseries1
ChartView{
   id: chartObj

   Component.onCompleted:{
    lSeries1 = chartView.createSeries(ChartView.SeriesTypeLine,         
               qsTr("Chamber"), timeAxis, tempAxis)
   }
}
onDataArrival:{
   lseries1.append(x,y)
}

[JonB]

@Beemaneni-Bala-0
Well, 120,000 points is quite a lot to plot and for the user to look at.

In widgets rather than QML at least I think it has been noted that appending one point at a time is "slow". Don't know if you can "buffer" the points and add multiple ones at a time. Otherwise think about "thinning" your data points, do you think a user can visualize 40,000 points lying on a line?

[Bob64]

@Beemaneni-Bala-0 I originally used one of the examples as my model for charts - I think it was called Oscilloscope. In this, the update of the series is done in the C++ backend and, as JonB said, multiple points are added in one go.

The basic idea is that the series updating C++ function is exposed to QML, and accepts a QXYSeries* parameter. You simply pass your QML series lseries1 to the QML-exposed function and on the C++ side it automatically sees it as a pointer to the QXYSeries C++ type.

Instead of having a "data arrival" signal for each point, append to a buffer in your back end and periodically notify the QML that new data is available - you can use a timer so that your update frequency is not excessive. The QML signal handler for the new data signal then calls the function I mentioned above.