Make test for gnuplot work with empty strings.

When I launch fio_generate_plots on a system where gnuplot is not installed I
get this error :

$ fio_generate_plots test
Making bw logs
/usr/bin/fio_generate_plots: 42: /usr/bin/fio_generate_plots: -: not found

That's because the test checking whether gnuplot is installed is failing
because of an empty variable :
GNUPLOT=$(which gnuplot)
if [ ! -x $GNUPLOT ]; then
        echo You need gnuplot installed to generate graphs
        exit 1
fi

Indeed the command "test -x" is exiting with code 0.

To correct this we should enclose the variable with quotes :
if [ ! -x "$GNUPLOT" ]; then

Then the script is going into the test properly :
$ fio_generate_plots test
You need gnuplot installed to generate graphs

I copied problem description by Hervé from the Debian bug report below.

Fixes:
Bug#700580: /usr/bin/fio_generate_plots: -: not found
http://bugs.debian.org/700580

Reported-By: Hervé Werner <hwerner@score-md.com>
Tested-By: Hervé Werner <hwerner@score-md.com>
Tested-By: Martin Steigerwald <ms@teamix.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
fio2gnuplot: Fixing typo when inserting fake data

Commit f9846c392d176582916c39b1925a436e79f23de1 introduced fake data
when matched files doesn't have the exact same amount of traces. This
patch introduced a typo that prevented the int comparison to work
properly.
fio2gnuplot: Print line if int() conversion fails

When data aren't properly formated the int() conversion could fails but
doesn't print the impacted line which is pretty difficult to estimate
where the bug is.

This patch simply reports the line that cause the trouble.
fio2gnuplot: Adding time selection options

It's could be very handy being able to extract a portion of a trace to
plot it.

This patch adds two new options to select the starting time and the end
time were the logs shall be considered for plotting.
fio2gnplot: Renaming file variable to myfile

'file' could be reserved on some modules or in the python.
It's better to rename it to avoid troubles.
fio2gnuplot: Fixing plotting issues on shorten files

fio2gnuplot was mixing files when some got shorten.

The position inside the array was used to remember what file was read
leading to mistakes once some values got ignored.

As a fix, we shall count the index for every single line we read and add
to the time & perf we keep. While writing data to the temporary files,
the extraction of the index insure we write to the proper file.

Before this patch, we observed at the end of the traces some stupid artefacts like loops... was
looking like a 4years old kid drawing...

That's now fixed.
fio2gnuplot: Plotting average value for compare graphs

When comparing in 2D several plots, it's pretty useful to have the
average value printed in the same graph.
fio2gnuplot: Display average value in label

When plotting 2D or mathematical graphs, let's show the numerical value
of the average value to ease the reading.
iolog: Logging blocksize in IOPS traces

When saving bandwidth's with write_bw_log option, the last column
provides the blocksize used during the run.

When doing the same with write_io_log option, the last column was always
set to 0 which is confusing when reading the log file later.

This patch put both write_bw_log & write_io_log using the same semantic
by reporting the blocksize for every sample.