[Kst] Avoid negative times on x axis

[Mark Dammer]

Maybe it is too obvious for me to see the solution, but I have the
following problem:
The X-Axis in my plot is created from a column in the datafile that has
time in Unix/C-Time format. The plot now shows the timeline, but starts
counting the minutes at the first even full minute what means that there
is "negative time" before the [0] mark. What I want is that the axis
starts at zero with the time in the axis description being the actual
time of the first sample and then showing ticks at two minute intervals
relative to the start.
thanks, Mark