understanding QwtPlot::transform()

[b.mourat]

Hi! I'm trying to draw Point and Figure chart (see attachment). As you can see on this chart the size of the symbols is too small for this scale. I want to compute the size of the symbols depending on the number of columns on the chart. I'm trying to use QwtPlot transform() function, but i dont understand how it works. For example it returns the same result in both of this cases:

current_plot->transform(QwtPlot::xBottom, 20.0);
current_plot->transform(QwtPlot::xBottom, 21.0);

I'm new to qwt, can anyone help???