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>
diff --git a/graph.h b/graph.h
index cb1e2d5..0099525 100644
--- a/graph.h
+++ b/graph.h
@@ -85,5 +85,7 @@
 extern const char *graph_find_tooltip(struct graph *g, int x, int y);
 extern int graph_contains_xy(struct graph *p, int x, int y);
 
+extern void graph_set_base_offset(struct graph *g, unsigned int base_offset);
+
 #endif