How to plot graphs using Qcustomplot in the main thread?

Question

I would like to know how to plot the serial port data in the Qcustomplot library to keep updating. I thank you for understanding.

void serialThread::run(){

mutex.lock(); 

 if(serial->waitForReadyRead(1)){

  QByteArray data = serial->readAll();

     qDebug () << data;

 }
   mutex.unlock();
 }

Answer 1

Using Qcustomplot to plot:

The class QCustomPlot inherits from QWidget and so always operates on the main thread. An example taken directly from documentation:

// generate some data:
QVector<double> x(101), y(101); // initialize with entries 0..100
for (int i=0; i<101; ++i)
{
  x[i] = i/50.0 - 1; // x goes from -1 to 1
  y[i] = x[i]*x[i]; // let's plot a quadratic function
}
// create graph and assign data to it:
customPlot->addGraph();
customPlot->graph(0)->setData(x, y);
// give the axes some labels:
customPlot->xAxis->setLabel("x");
customPlot->yAxis->setLabel("y");
// set axes ranges, so we see all data:
customPlot->xAxis->setRange(-1, 1);
customPlot->yAxis->setRange(0, 1);
customPlot->replot();

Where the customPlot is a QCustomPlot previously created.

---

On how to plot out the main thread:

For, in my opinion, a design flaw of the library, the class responsible for displaying the graph on the screen is the same responsible for rendering the graph itself. That is, at the same time that it performs intensive processing, it inherits from QWidget, that cannot be created outside the main thread.

That way you have some not so good options:

1. Create an external program that receives parameters on the command line and then render the chart using the widget. Note that you don’t need to call the function exec() application, then this process ends immediately after rendering and saving the chart. Then, from your main program, run this by passing the parameters you need and waiting for termination. You can use QProcess for this. Note that unfortunately we will still need to create a QApplication there, causing the loading of the entire Qt display structure.

2. Take a more aggressive attitude and edit the library by removing dependency from the QWidget. It works, but it’s not quite ideal and will cause a lot of work unrelated to your program itself. In addition to the risk of causing new bugs, and having a whole rework when releasing new versions.

If you are going to take Path 2, I suggest you contribute your changes to their repository, improving the library as a whole and taking the weight out of you maintaining the code.