x-axis Label in hh:mm:ss format


I have time data stored in the format hh:mm:ss and would like to plot it, having about 4 to 6 ticks labeled in this format.
What I’m doing at the moment is to first convert my time to seconds and plot y as function of yime in seconds.

Here the first question: Is this the way to go or is there a function I
missed which allows to directly plot from the former format?

Now I’d like to have the labels to be displayed in the hh:mm:ss format,
in a way that it automatically adopts when zooming/dezooming. Without
zooming I figured out that it is quite easy to use the xticks method,
but is there a possibility to write a function which automatically
readadopts the tick spacing?
If not, I found in the manual the dateformatter class, but if I
understand correctly, I first have to transfer the time to strftime? Or
can I directly use this class using the time starting from midnight?

Thanks for any response,