#12 dataset file format documentation

[Ulrich Hauser-Ehninger]

There seems to be no specific place with a thorough definition of the filebased source dataset format.
What I could find are very simple examples like:
<Qucs Dataset="" 0.0.19="">
<indep time="" 2="">
+0
+1
</indep>
<dep V1="" time="">
+0
+15
</dep>
But this does not explain anything about certain topics:
Is it only possible to have a linear transition between data points? No Sine, Cosine or Spline or other segment types?
What happens at the end of the definition? It seems, that the last transition is extrapolated until the end of the simulation time.
A documentation would be very helpful.