1. Have enough space to do so
2. Don't mess with the default settings

In my case, I had the axis tick count set pretty high which was causing the labels to squish together. If you leave the tick counts at their default, or let Qt decide, it should work just fine.

Eventhough this thread is quite old, I faced a similar issue with this truncated axis labels. I found a workaround which is working for me by removing and adding the axes again to the chart.

Here is the example code I use (precondition: axes must be added once to chart and series before):

    auto axes = chart()->axes();
    QHash<QAbstractSeries*, QList<QAbstractAxis*>> seriesAxes;
    for (auto serie : series) {
        auto attachedAxes = serie->attachedAxes();
        for (auto axis : attachedAxes) {
            if (axes.contains(axis)) {
                auto &serieAxes = seriesAxes[serie];
                serieAxes.append(axis);
            }
        }
    }
    for (auto axis : axes) {
        chart()->removeAxis(axis);
        auto alignment = axis->alignment();
        chart()->addAxis(axis, alignment);
    }
    QHashIterator<QAbstractSeries*, QList<QAbstractAxis*>> iSeriesAxes(seriesAxes);
    while (iSeriesAxes.hasNext()) {
        auto serie = iSeriesAxes.next().key();
        auto axes = iSeriesAxes.value();
        for (auto axis : axes) {
            serie->attachAxis(axis);
        }
    }

Cheers,
Reto

I am facing a very similar issue. I do not have that many charts to draw, yet too many labels on a Y axis. The QChart are so badly shaped, that it seems it is impossible to easily fix it. A first option, is to change the pixel size of the font used to draw the labels in the resize event.

void yourCharView::resizeEvent(QResizeEvent *event)
{
    if(series.size() > 0) {
        QFont font = this->axis_y_stack->labelsFont();
        int pixel_size = event->size().height() / series.size() / 3;
        font.setPixelSize(pixel_size);
        this->axis_y->setLabelsFont(font);
    }

    QChartView::resizeEvent(event);
}

(this->y_axis beeing your y axis)

But it might prove insufficient, as in my case.

Labels are truncated in verticalaxis.cpp (for the vertical axes) :

QRectF boundingRect;
// don't truncate empty labels
if (text.isEmpty()) {
       labelItem->setHtml(text);
 } else {
       qreal labelHeight = (axisRect.height() / layout.count()) - (2 * labelPadding());
       QString truncatedText = ChartPresenter::truncatedText(axis()->labelsFont(), text,
                                                                 axis()->labelsAngle(),
                                                                 availableSpace,
                                                                 labelHeight, boundingRect);
       labelItem->setTextWidth(ChartPresenter::textBoundingRect(axis()->labelsFont(),
                                                                    truncatedText).width());
       labelItem->setHtml(truncatedText);
}

The truncation code lies here :

QString ChartPresenter::truncatedText(const QFont &font, const QString &text, qreal angle,
                                      qreal maxWidth, qreal maxHeight, QRectF &boundingRect)
{
    QString truncatedString(text);
    boundingRect = textBoundingRect(font, truncatedString, angle);
    if (boundingRect.width() > maxWidth || boundingRect.height() > maxHeight) {
        // It can be assumed that almost any amount of string manipulation is faster
        // than calculating one bounding rectangle, so first prepare a list of truncated strings
        // to try.
        static QRegularExpression truncateMatcher(QStringLiteral("&#?[0-9a-zA-Z]*;$"));
........
        static QLatin1String ellipsis("...");
........
        // Default to "..." if nothing fits
        if (bestIndex == count) {
            boundingRect = textBoundingRect(font, ellipsis, angle);
            truncatedString = ellipsis;
        } else {
            truncatedString = testStrings.at(bestIndex);
        }
    }
    return truncatedString;
}

I can't think of any way to interfere with this and avoid turning my nice labels into three dots...

Any input from anyone else ? We can't even change the labelPadding() !!!!

 inline qreal labelPadding() const { return qreal(4.0); }

=> I will hide the presenter labels and draw the labels myself, overriding the paintEvent.

Cheers

Côme