tickmode for durations

[vallsv]

It could be interesting to add an extra tick mode for durations if it is not too complex to implement, as we already have a tickmode for time series.

Right now i have to use:

# NOTE: There is no plot axis for duration
# the elapse time will be displayed in 1970
# the UTC force to display 0 at 0
axis.setTickMode(axis_mdl.TickMode.TIME_SERIES)
axis.setTimeZone("UTC")

It works but

• labels on the negative side of the axis are displayed, which is surprising
• for long scans maybe some labels will be displayed with "1970" text, or number of days or months like 01