benchlog: Add gnuplot support

generate plot image of benchlog by gnuplot (if system has gnuplot)

Change-Id: Ib7d94313bf061c008267f30b339f92f4ea336b3a
Reviewed-on: https://code-review.googlesource.com/3150
Reviewed-by: Paul Wankadia <junyer@google.com>
1 file changed