To be clear what I want to achieve I provide an easy example which points out what is not working yet.
This code:
\begin{ganttchart}[hgrid,vgrid]{1}{7}
\gantttitlelist{1,...,7}{1} \\
\ganttbar{}{1}{3.5} \\
\ganttbar{}{4.2}{5} \\
\ganttmilestone{}{6.2}
\end{ganttchart}
leads to this chart:
As you can see, each element (bar, milestone) has been aligned to a time slot. But I want these elements to start/end at exactly that place which I defined, so e.g. the first bar should end in the middle between two slots, the milestone should be at 6.2 (and not 6.0) and so on.
How can I achieve this?