QLineSeries and QDateTimeAxis - Chart doesn't display values

Question

I am trying to achieve something like https://doc.qt.io/qt-5.9/qtcharts-datetimeaxis-example.html .

In a method I have this code

QDateTime past = QDateTime::currentDateTime().addMonths(-10);
QDateTime now = QDateTime::currentDateTime();
qreal pastvalue = 4;
qreal nowvalue = 4;
axisY = new QValueAxis();
axisX= new QDateTimeAxis();
chart = new QChart();
series = new QLineSeries();
/*Y*/
axisY->setLabelFormat("%i");
axisY->setTitleText("Numero dispositivi");
axisY->setMin(0);
axisY->setMax(5);
/*X*/
axisX->setTickCount(2);
axisX->setMin(past);
axisX->setMax(now);
axisX->setFormat("dd-MM-yyyy h:mm:ss"); 
/*series/*
series->attachAxis(axisX);
series->attachAxis(axisY);
series->append(past.toSecsSinceEpoch(),pastvalue);
series->append(past.toSecsSinceEpoch(),nowvalue);
/*chart*/
chart->legend()->hide();
chart->setTitle("Dati filtrati durante il periodo temporale");
chart->addAxis(axisY, Qt::AlignLeft);
chart->addAxis(axisX, Qt::AlignBottom);
chart->addSeries(series);

chartView = new QChartView(chart);
ui->verticalLayout->addWidget(chartView);

I don't understand why values are not displayed.

I can't figure out what's wrong.

Answer 1

Your code has the following errors:

• You have to use toMSecsSinceEpoch() instead of toSecsSinceEpoch().
• You have to add the axis to the QChart first before attaching it to the series.

• change series->append(past.toSecsSinceEpoch(),nowvalue); to series->append(now.toMSecsSinceEpoch(),nowvalue);

• I recommend you to set the ticks so that only the whole points are shown since, for example, the inappropriate value will be displayed on the vertical axis, or set as floating with a certain number of decimal places, in the following image I show the result with %i:

As we can see in the previous image, 4 is very close to 3 and farther than 5, instead of being equidistant, so in the solution I propose, you will use %.2f:

QDateTime past = QDateTime::currentDateTime().addMonths(-10);
QDateTime now = QDateTime::currentDateTime();
qreal pastvalue = 4;
qreal nowvalue = 4;
axisY = new QValueAxis();
axisX= new QDateTimeAxis();
chart = new QChart();
series = new QLineSeries();
/*Y*/
axisY->setLabelFormat("%.2f");
axisY->setTitleText("Numero dispositivi");
axisY->setMin(0);
axisY->setMax(5);
/*X*/
axisX->setTickCount(2);
axisX->setMin(past);
axisX->setMax(now);
axisX->setFormat("dd-MM-yyyy h:mm:ss");
/*series*/

series->append(past.toMSecsSinceEpoch(),pastvalue);
series->append(now.toMSecsSinceEpoch(),nowvalue);
/*chart*/
chart->legend()->hide();
chart->setTitle("Dati filtrati durante il periodo temporale");
chart->addAxis(axisY, Qt::AlignLeft);
chart->addAxis(axisX, Qt::AlignBottom);
chart->addSeries(series);
series->attachAxis(axisX);
series->attachAxis(axisY);

chartView = new QChartView(chart);
ui->verticalLayout->addWidget(chartView);