gfio: graphing updates

gfio: graphing updates

- Don't draw tickmark labels for an empty graph
- Remove Y-axis label. It was a bit awkward and hard to read.
- Ensure that we always put proper units on the Y-axis graph.
- Ensure that we always include the top most Y-axis tickmark
  and make that the ceiling, not the highest Y value. It's hard
  to visually understand a graph that is drawing somewhere above
  the last labeled tickmark.
- Fix rounding errors in graph.c

Signed-off-by: Jens Axboe <axboe@kernel.dk>