Drawing a waveform using QCustomPlot

[marcelnitan]

On stackoverflow I came across a piece of code that will use QCustomPlot to draw a waveform from an audio. Trying to adapt the code to my needs I came across an issue where the plot lines were drawn on the whole plot as seen in the screenshot bellow.

What I changed is I've made the script use an already existing AudioDecoder here.

Could someone help me on what's going wrong? Thanks.

Waveform.h

#ifndef WAVEFORM_H
#define WAVEFORM_H


#include "qcustomplot.h"
#include <QAudioBuffer>

class QAudioDecoder;

class Waveform : public QCustomPlot
{
    Q_OBJECT

public:
    Waveform(QWidget *parent = Q_NULLPTR);
    ~Waveform();
    void setBuffer(QAudioBuffer buffer);
    void plot();
    void reset();

private:
    qreal getPeakValue(const QAudioFormat& format);
    QVector<double> samples;
    QCPGraph *wavePlot;
};
#endif // WAVEFORM_H

Waveform.cpp

#include "waveform.h"

#include <QAudioDecoder>

Waveform::Waveform(QWidget *parent)
    : QCustomPlot(parent)
{
    wavePlot = addGraph();
    setMinimumHeight(250);
}

Waveform::~Waveform()
{
}

void Waveform::setBuffer(QAudioBuffer buffer)
{
    qreal peak = getPeakValue(buffer.format());
    const qint16 *data = buffer.constData<qint16>();
    int count = buffer.sampleCount() / 2;
    for (int i=0; i<count; i++){
        double val = data[i]/peak;
        samples.append(val);
    }
}

void Waveform::plot()
{
    QVector<double> x(samples.size());
    for (int i=0; i<x.size(); i++)
        x[i] = i;
    wavePlot->addData(x, samples);
    yAxis->setRange(QCPRange(-1, 1));
    xAxis->setRange(QCPRange(0, samples.size()));
    replot();
}

void Waveform::reset()
{
    samples.clear();
}

qreal Waveform::getPeakValue(const QAudioFormat &format)
{
    qreal ret(0);
    if (format.isValid()){
        switch (format.sampleType()) {
            case QAudioFormat::Unknown:
            break;
            case QAudioFormat::Float:
                if (format.sampleSize() != 32) // other sample formats are not supported
                    ret = 0;
                else
                    ret = 1.00003;
            break;
            case QAudioFormat::SignedInt:
                if (format.sampleSize() == 32)
#ifdef Q_OS_WIN
                    ret = INT_MAX;
#endif
#ifdef Q_OS_UNIX
                    ret = SHRT_MAX;
#endif
                else if (format.sampleSize() == 16)
                    ret = SHRT_MAX;
                else if (format.sampleSize() == 8)
                    ret = CHAR_MAX;
                break;
            case QAudioFormat::UnSignedInt:
                if (format.sampleSize() == 32)
                    ret = UINT_MAX;
                else if (format.sampleSize() == 16)
                    ret = USHRT_MAX;
                else if (format.sampleSize() == 8)
                    ret = UCHAR_MAX;
            break;
        default:
            break;
        }
    }
    return ret;
}