perf timechart: Fix wrong SVG height

perf timechart: Fix wrong SVG height

If we call perf timechart with -p 0 arguments, it means we don't want
any tasks related data. It works, but space for tasks data is reserved
in the generated SVG. Remove this unused empty space via passing 0 as
count to the open_svg.

Signed-off-by: Stanislav Fomichev <stfomichev@yandex-team.ru>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lkml.kernel.org/r/1390217980-22424-2-git-send-email-stfomichev@yandex-team.ru
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>