Plot only part of QwtCurve in real time plot

Question

I have a QVector in which I'm constantly appending data so I can plot on a QwtPlot. But with a high frequency I guess the vector becames too large and the program crashes.

My question is, how can I create a QwtCurve which is only beggining in some point of time, bacause the time that has already passed is not necessary in the vector as it was already plotted.

here's my code:

QVector<double> xv;
QVector<double> cv1;
QVector<double> cv2;

as global variables,

void gui::process_new_info(QByteArray array)
{
    int v = 0;

    double f = ui->frequency->value();

    xv.append(sizeof_array/f);

    char *buf = array.data();

    if (ui->ecgPlux->isChecked() == true || ui->channel_1->currentIndex() != 0)
    {
        cv1.append(buf[1]);

        QwtPlotCurve *curve1 = new QwtPlotCurve;
        curve1->attach(plot_all[0]);
        curve1->setData(xv,cv1);
        curve1->setPen(QPen(Qt::blue,1));
        plot_all[0]->replot();

        QwtPlotCurve *curve2 = new QwtPlotCurve;
        curve2->attach(plot[0]);
        curve2->setData(xv,cv1);
        curve2->setPen(QPen(Qt::blue,1));
        plot[0]->replot();
    }

    if (ui->xyzPlux->isChecked() == true || ui->channel_2->currentIndex() != 0)
    {
        cv2.append(buf[2]);

        QwtPlotCurve *curve3 = new QwtPlotCurve;
        curve3->attach(plot_all[1]);
        curve3->setData(xv,cv2);
        curve3->setPen(QPen(Qt::blue,1));
        plot_all[0]->replot();

        QwtPlotCurve *curve4 = new QwtPlotCurve;
        curve4->attach(plot[1]);
        curve4->setData(xv,cv1);
        curve4->setPen(QPen(Qt::blue,1));
        plot[1]->replot();
    }

    //printf ("%d ->", buf[0]);
    fprintf (data, "%d,", buf[0]);

    for (int i = 1; i < 9; i++)
    {
        v = buf[i];
        //printf ("%d,", v);
        fprintf (data, "%d,", v);
    }

    //printf ("\n");
    fprintf (data, "\n");

    sizeof_array++;
}

Great to see a Qwt question here but have you tried posting this on the Qwt forum? Last time I checked the author, Uwe Rathmann, was pretty good about getting back to people. — Matt Phillips, Mar 12 '13 at 15:46

score 2 · Accepted Answer · answered Mar 12 '13 at 22:33

QwtPlotCurve

http://qwt.sourceforge.net/class_qwt_plot_curve.html

inherits from QwtPlotSeriesItem

http://qwt.sourceforge.net/class_qwt_plot_series_item.html

There is a warning in setData

The item takes ownership of the data object, deleting it when its not used anymore.

It probably isn't the fact that you are growing too big... it may be that you are accessing something that Qwt deleted.

Run it in a debugger, and look at the stack trace for where it died, or put in a bunch of qDebug() lines to see where it is dying.

If it is something with the data being too large, you could pop off items off of the head of your vector, before setting the vector.

Hope that helps.

Plot only part of QwtCurve in real time plot

1 Answers1