For numbers greater than 30.000.000,0 the error when storing them in IEEE-754 format is greather than 1,0.
This is too much for my need and so I did it in a different way.
I store the value for the first x element and I work with small delta values (1, 2, 3, ...).
Then I use custom QwtDateScaleDraw to show the scale labels as I need.