Possibility to plot several tests on the same plot

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:
![multi_gp_example](https://f.cloud.github.com/assets/107720/187676/59d8ccee-7d7e-11e2-8e71-611c3ba20295.png)