Plot live data on QtChart

rockon209

Hello Everyone,

I want to plot live data which i am reading from a sensor and want to plot it on QtChart. I can read the data but dont know how to plot or pass the values to QtChart. Please help, thanks in advance

SGaist

Hi,

Do you have something like the QtChart QMLOscilloscope in mind ?

rockon209

@SGaist

Yes i want to use only Qtchart, I havent used QML till now so dont know how it works. How can i plot it in Qtchart?
I want somthing like this just instead of microphone as input i want my sensor as the input see the link below
http://doc.qt.io/qt-5/qtcharts-audio-example.html

jsulm

@rockon209 Then use your sensor as input.

rockon209

@jsulm
Ya that obvious but how i can do that i was not able to pass the values form sensor to QtChart i can read the data from sensor and i am getting it in a varaible now how to pass it to Qtchart?????
For e.g.
read the data and store it in a double variable
double tempSensor= readSignal Sensor(tempChannel);

jsulm

@rockon209 like in the example:

qint64 XYSeriesIODevice::writeData(const char * data, qint64 maxSize)
{
    qint64 range = 2000;
    QVector<QPointF> oldPoints = m_series->pointsVector();
    QVector<QPointF> points;
    int resolution = 4;

    if (oldPoints.count() < range) {
        points = m_series->pointsVector();
    } else {
        for (int i = maxSize/resolution; i < oldPoints.count(); i++)
            points.append(QPointF(i - maxSize/resolution, oldPoints.at(i).y()));
    }

    qint64 size = points.count();
    for (int k = 0; k < maxSize/resolution; k++)
        points.append(QPointF(k + size, ((quint8)data[resolution * k] - 128)/128.0));

    m_series->replace(points);
    return maxSize;
}

in your case instead of char* data you have a double.

rockon209

@jsulm
ya but i was not able to understand the code, what i am doing in my code is i am reading every on second from the sensor and i store it to the variable and then immediately i want to pass it to Qtchart and again read the sensor after one second how i can so it with the above code i new to this data acquisition

jsulm

@rockon209 Change the code from the example, so it matches your requirements:

qint64 XYSeriesIODevice::writeData(const double data, qint64 maxSize)
{
// Adjust here as well as needed
}

Then call writeData every time you get new value from sensor.
To read sensor every second use QTimer.

rockon209

@jsulm
thanks for the reply but i have a question in the above code how the microphone data is pass to this writeData function if const char* data varaible has the info of the microphone data how is it pass to const char* data varaible??? I am not able to understand

jsulm

@rockon209 In the example they implement a device:

m_device = new XYSeriesIODevice(m_series, this);
m_device->open(QIODevice::WriteOnly);

m_audioInput->start(m_device);

XYSeriesIODevice is a QIODevice which is writing incoming char* data to the chart. The data is coming from m_audioInput.
In your case you do not need to implement a device: just use a QTimer to read your sensor each second and write the data to the chart in a similar way as in the example.

rockon209

@jsulm
ok thanks for the explanation
but it is confusing because in XYSeriesIODevice class have m_series as XYseries and widget class has m_series as Qlineseries
and tried your above suggestion i am getting the following error
error: invalid types 'double[int]' for array subscript
points.append(QPointF(k + size, ((quint8)tempSensor[resolution * k] - 128)/128.0));

jsulm

@rockon209 said in Plot live data on QtChart:

QPointF

Can you show the whole method?

rockon209

@jsulm
with timer i call the following function every second
void test:: readTemp()
{
double tempSensor= readSignal Sensor(tempChannel);
writeData();
}
void test::writeData()
{
qint64 range = 2000;
QVector<QPointF> oldPoints = m_series->pointsVector();
QVector<QPointF> points;
int resolution = 4;
qint64 maxSize;

if (oldPoints.count() < range) {
    points = m_series->pointsVector();
} else {
    for (int i = maxSize/resolution; i < oldPoints.count(); i++)
        points.append(QPointF(i - maxSize/resolution, oldPoints.at(i).y()));
}

qint64 size = points.count();

for (int k = 0; k < maxSize/resolution; k++)
    points.append(QPointF(k + size, ((quint8)tempSensor[resolution * k] - 128)/128.0));

m_series->replace(points);

//return maxSize;
}

jsulm

@rockon209 said in Plot live data on QtChart:

double tempSensor= readSignal Sensor(tempChannel);

this is not going to work - you store the temperature in a local variable!

Also, this is wrong if tempSensor isn't an array:

points.append(QPointF(k + size, ((quint8)tempSensor[resolution * k] - 128)/128.0));

As I said you need to adjust the code for your needs...

rockon209

@jsulm
then how to make array as i am reading from the sensor every second and getting only one value.

jsulm

@rockon209 You don't need any array:

void test:: readTemp()
{
writeData( readSignal Sensor(tempChannel));
}

void test::writeData(const double tempSensor)
{
qint64 range = 2000;
QVector<QPointF> oldPoints = m_series->pointsVector();
QVector<QPointF> points;
int resolution = 4;
qint64 maxSize;

if (oldPoints.count() < range) {
    points = m_series->pointsVector();
} else {
    for (int i = maxSize/resolution; i < oldPoints.count(); i++)
        points.append(QPointF(i - maxSize/resolution, oldPoints.at(i).y()));
}

qint64 size = points.count();
points.append(QPointF(k + size, ((quint8)tempSensor - 128)/128.0));
m_series->replace(points);
}

rockon209

@jsulm
i am getting following error and the application is shutting down
terminate called after throwing an instance of 'std::bad_alloc'
what(): std::bad_alloc

jsulm

@rockon209 Please debug you app to see where it crashes...

rockon209

@jsulm
when i read the sensor data it crashes

jsulm

@rockon209 Then find out why and fix it...