qscrollbar (not a qscrollarea) setValue not working

Question

I am working on a qwtPlot and have implemented custom scrollbars to illustrate the position of things on the plot when zooming in (in regards to the whole thing - so basically the percentage).

Now, everything works fine, apart from the moment, when I do any zooming or panning right at the beginning (or simply when I see the whole plot and then I want to zoom in).

This is a slot I am using to refresh the appearance of the scrollbar:

void ScrollHorizontal::refreshAfterChanges() {
setValue(myPlot->getLowerBound(QwtPlot::xBottom));
setPageStep(myPlot->get_X_delta());
setSingleStep(myPlot->get_X_delta()/10);
setMaximum(myPlot->dataFromSources->lastTimeValMicro()-pageStep());  
update();
printV("HorizontalScroll::setValue",myPlot->getLowerBound(QwtPlot::xBottom));
printV("value", value());
printV("pageStep", pageStep());

in the constructor I set the maximum to 0 (just in case, but it doesn't change anything)

The last 3 lines print out some values useful for debugging. What I found out thanks to them is that the slot is executed correctly, but the setValue(int) function doesn't work as I would expect it to:

//printed values right after starting the program
HorizontalScroll::setValue=0
value=0
pageStep=7320

//printed values after using the zoomer once
HorizontalScroll::setValue=956.316
value=0
pageStep=2225

Then, when I move the plot a tiny bit, e.g. zoom it 1.1 times, the setValue works properly and value() return the same thing I set. But if I go to the 100% view (the starting point) I get the same problems again.

Just to illustrate it, here are some screenshots:
http://tinypic.com/view.php?pic=2znph7s&s=8#.UvNkDoZhauY
(100% view, right before zooming in)

http://tinypic.com/view.php?pic=ke7nn9&s=8#.UvNkYIZhauY
(badly set qscrollbar)

Answer 1

Ok - problem was caused by the line

setMaximum(myPlot->dataFromSources->lastTimeValMicro()-pageStep());  

It was setting the maximum to 0 sometimes. why? I wanted the slider to take up the whole length of the scrollbar so that at the beginning when the plot was shown in 100% view, you couldn't scroll it and I could achieve that by setting the pagestep to my plot's maximum value and the maximum to 0.

But that caused the setValue(int)problem - you can't set a value bigger than the maximum.

So what finally worked for me is:

double valueToSet=myPlot->getLowerBound(QwtPlot::xBottom);
if(valueToSet>maximum())
    setMaximum(valueToSet);

setValue(myPlot->getLowerBound(QwtPlot::xBottom));
setPageStep(myPlot->get_X_delta());
setSingleStep(myPlot->get_X_delta()/10);
setMaximum(myPlot->dataFromSources->lastTimeValMicro()-pageStep());
update();