benchmark: allow to add lines to scatter plots
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
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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