Label on QwtDateScaleDraw vanishes once and get back, strange behavior

[HappyCoder]

Hello,

in my application it is possible to switch on the x-axis between

QwtLinearScaleEngine/HourMinuteSecondsScaleDraw (relative time starting from zero)

and

QwtDateScaleEngine/DateScaleDraw (absolute time starting at UTC Offset).

Our time values is a double in seconds and the lables are overloaded
(but that is not the source for this behavior. Without them just wrong labels are shown
but it happens also. I add these only because the labels are not original Qwt if someone
asking for that.)

QwtScaleDraw for QwtLinearScaleEngine:

class HourMinuteSecondsScaleDraw: public QwtScaleDraw
{
public:
    virtual QwtText label( double value ) const
    {
        QTime n(0,0,0);
        QTime t;
        t = n.addSecs( value );
        return t.toString(Qt::ISODate);
    }
};

class HourMinuteSecondsScaleDraw: public QwtScaleDraw
{
public:
    virtual QwtText label( double value ) const
    {
        QTime n(0,0,0);
        QTime t;
        t = n.addSecs( value );
        return t.toString(Qt::ISODate);
    }
};

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QwtScaleDraw for QwtDateScaleEngine:

class DateScaleDraw: public QwtDateScaleDraw
{
public:
    DateScaleDraw( Qt::TimeSpec spec) : QwtDateScaleDraw (spec)
    {
    }
 
    virtual QwtText label( double value ) const
    {
        return QwtDateScaleDraw::label( value*1000 );
    }
};

class DateScaleDraw: public QwtDateScaleDraw
{
public:
    DateScaleDraw( Qt::TimeSpec spec) : QwtDateScaleDraw (spec)
    {
    }

    virtual QwtText label( double value ) const
    {
        return QwtDateScaleDraw::label( value*1000 );
    }
};

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1.) when i start a measurement with QwtLinearScaleEngine/HourMinuteSecondsScaleDraw
everything is fine and the lables are always visible at any time.
01 QwtLinearScaleEngine ISODateScaleDraw.jpg

2.) when i start a measurement with QwtDateScaleEngine/DateScaleDraw for the first
measurement data the labels vanishes and return when 2 measurment points in out data.
02 QwtDateScaleEngine and DateScaleDraw.jpg

It is hard to find a good entry point for debugging or why this happens. Can anybody
give me a hint why the lables for QwtDateScaleDrawEngine vanishes shortly?

Thx

[Uwe]

When working with the QwtDateScale classes values need to be dates: stored as ms since Epoch ( see http://qwt.sourceforge.net/class_qwt_date.html#details )
But to me it looks like you are looking for something like this: http://sourceforge.net/p/qwt/feature-requests/56

Uwe