changes to default tickformat

- set default tickformat depending on single/multichannel
- remove minutes from singlechannel tickformat (@bonaime, did you add
  minutes here because of the strange aligning, with :00 minutes not at
  the left side? I think that should be fixed by becdfb51f90c2bbb. You
  can check if the labels are matching the data with "-v" option that
  prints information on the plotted stream since ffb7b2168dbf2f)