fio2gnuplot: Adding 2D graphs for each fio trace

fio2gnuplot: Adding 2D graphs for each fio trace

When tracing multiple files, the 3D graph is useful to understand the
behavior of a group of disks but makes difficult to understand how a
particular disk behave during the test.

This patch does add three 2D plots for each fio trace :
- the 2D Raw tracing
This plot is the exact plotting of the raw values reported by fio.
It's very precise but hard to read when having many many traces

- the 2D Smooth tracing
This plot is a smooth version of the raw tracing. It tries to keep as
much as possible a tracing close to the raw one but makes it more
readable by aggregating points.

- the 2D Trend tracing
This plot is trying to setup a trend of the global performance. It
aggregates many points and hide peaks. It only consider at giving a very
easy to understand plotting on how the bench was performing.
It's much more easier to read but could be misleading if raw or smooth
tracing didn't got read before.