T: some work on XY plot

- change paint function:
  when the XY plot is resized we scale. Previously
  the last range was put in the middle of the new range
  (say when we increase the size).
  We may get aspect ratio changes if the resize is not identical
  vertically and horizontally, but I think this behaviour is more
  'natural'.
- fix a bug:
  the last horizontal tick label was printed to far on the right,
  out of the bouding box of the XY plot. This is not totally fixed
  in the case the label is larger than the XY plot. Now the part
  out of the bounding box will be printed on the left. No big deal,
  make the plot big enough. (Before, even if big enough you had a
  problem.)
- add a new vertical tick display, to be used for throughput mostly.
  See in enb.c the difference between 'input signal' and throughput
  plots (those throughput plots will come in later commits).