Qwt 6.3.0 (from qwt-code-r2921-trunk) compilation fails on Ubuntu 18.04.

Have a look at: http://qwt.sourceforge.net/class_qwt_series_data.html and http://qwt.sourceforge.net/class_qwt...05dfae2d1668e6

The implementation could be like this:

Code:

---

virtual QPointF YourData::sample( size_t  index )  const
{
    qreal currentX = ...;
    qreal x = ...;
    qreal y = ...;
 
    return QPointF( x - currentX, y );
}

---

This translates "ms since being started" into "ms before now" and you can set a range of [-10000, 0] to always see the samples of the last 10 seconds.
Be careful, that calculating x/y/currentX is not expensive - in case you need to render many points ( many more than 400 isn't ).

In case of having many points outside of the 10s ( or whatever your frame is ) it might be worth to enable polygon clipping.

Uwe

Uwe, many thanks, I will try it.

Uwe,

Code:

---

virtual QPointF YourData::sample( size_t  index )  const
    {
        ...
        qreal currentX = ...;
        ...
    }

---

Are you sure that this will not a bootleneck too? Because for this situation we need to call the QDateTime::currentDatetime() each time for each sample...

Quote:

---

Originally Posted by kuzulis

Because for this situation we need to call the QDateTime::currentDatetime() each time for each sample...

---

Of course you have to calculate your reference time only once and not for each call of sample().
So what about storing QDateTime::currentDatetime() as reference time inside of the data object, whenever you receive a new set of samples.

You could also rely on the fact, that iterating over samples always starts at index 0 and always update the reference time on this one.
But I wouldn't recommend to do this, as you always have a delay until the plot is updated and then you have a gap between the data and the reference time.

Uwe

Uwe,

So, your idea is to calculate the reference time like this:

e.g.:

Code:

---

...
 
MySeriesData : public QwtSeriesData
{
...
    MySeriesData (qint64 reftime, const QVector<QPointF> &samples) 
        : m_reftime(reftime)
        , m_samples(samples)
    {
    }
 
    QPointF sample(size_t  index)  const
    {
        ...
        ...
        return QPointF(x - m_reftime, y);
    }
 
private:
    qint64 m_reftime= 0;
    QVector<QPointF> m_samples;
}
...
 
// And then...
const auto reftime = QDateTime::currentMSecsSinceEpoch();
curve->setData(new MySeriesData(reftime, samples));
..

---

?

Well, m_reftime is a qint64, while the samples are doubles. But as long as they match, why not.

Uwe