fsperf: Add "set yrange [0:]" to make easier to compare

Now, all graphs are using autoscale for yrange. But it may be confusable
to compare. So, by default, this sets "set yrange [0:]" for some graphs.

Signed-off-by: OGAWA Hirofumi <hirofumi@mail.parknet.co.jp>
fsperf: Change "Block Accessses" to "I/O Request Position"

Signed-off-by: OGAWA Hirofumi <hirofumi@mail.parknet.co.jp>