QwtKnob and QwtLog10ScaleEngine

How can I make the QwtKnob object display a logarithmic scale from 10 to 1000 with the following markers on the scale 10, 30, 100, 300 and 1000?

So far this is what I have:

Code:

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QwtLog10ScaleEngine *engine = new QwtLog10ScaleEngine;
ui->d_knob->setScaleEngine( engine );
ui->d_knob->setRange(10.0, 1000.0 );

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And that results in the QwtKnob displayed with the markers 10, 100 and 1000 only.

Thanks in advance,

Help please!