benchmark: allow to add lines to scatter plots

[lundibundi]

Adds --add-lines option to scatter.R that will add lines between the points of the same category.

It was helpful for me to see a general trend with different options, so I added this.
Also, this is my first time working with R so please be thorough, maybe I've missed something.

Checklist

• make -j4 test (UNIX) passes
• documentation is changed or added (I'll change benchmark guide later it this is an acceptable change)
• commit message follows [commit guidelines]

/cc @nodejs/benchmarking