Possibility to plot several tests on the same plot

[motiejus]

This allows comparing benchmarks with each other. For example, it is
very convenient to visualise on the same graph how throughput changes
when {concurrent, N} changes.

For example, here is a Sphinx search engine throughput for my specific query for binary (light_bin_X) and SphinxQL (light_ql_X) protocols with X parallel workers:

Easy to construct:

 priv/gp_throughput.sh -k total -d tests/light_ql_1,tests/light_ql_2,tests/light_bin_1,tests/light_bin_2

[shino]

Options described here are applicable for both gp_throughput.sh and gp_latency.sh.
I think this feature is also useful for latency graph, isn't it?

[shino]

Thank you for contribution.
This feature is very useful and I immediately use this script to draw graphs for real benchmark.

This PR is sufficient as-is.
I added some comments in the "File Changed" tab for the better.

[motiejus]

@shino Good point. I will update the script to skip the directory name in case of single directory.

I never needed latency graphs though. But I think it should be fairly straightforward to do if someone needs it. In case I ever will, I will make a separate pull request.

[motiejus]

Fixed that naming issue and for completeness added the same functionality to gp_latencies.sh. @shino Thanks for feedback.

[slfritchie]

Note also that the file priv/compare.r will generate graphs that compare the results of two runs.

[motiejus]

@slfritchie thanks, I didn't know it exists. However, it allows comparing 2 graphs, so did not really fit my use case.

[shino]

@motiejus Thanks for changing latency script too! Merged to master.