hi,
QwtPlot::transform() is to be used to transform the (x,y) coordinates in the plot coordinates system to the global widget coordinates and the output is in units of screen points which are integers. Hence, both your code will give the same result because they are far to close. Try these:

current_plot->transform(QwtPlot::xBottom, 20.0);
current_plot->transform(QwtPlot::xBottom, 11.0);

and you will see the difference in output.

One way could be instead of using transform, you can directly set the size of the QwtSymbol depending on you column number, which is in plot-coordinates.

Kaustav.