Resize QwtSlider to full length of available space

Am very happy to have found qwt widgets, and am using QwtSlider. Using QGroupBox and QLayout classes, I am able to construct GUIs with widgets that resize themselves. But, this does not work with QwtSlider (unless I am doing something silly). I had a similar problem with another widget, and it was because it contained a pixmap, and pixmaps do not automatically resize, for reasons I understand. I assume the scale on the QwtSlider is also a pixmap, and this may be why I am having this problem. However, in the QwtSlider documentation, the picture shows beautiful sliders all the full length. What is the key to achieving that, please?

As often happens, I did find the source code for the pretty slider example: It is in the source files for sliderbox and slidertab in the examples/controls folder of the qwt distribution. I'm sure this will "show me the way".