add data index to legendFormatter data

[buck54321]

This adds the row index to the structure passed to the legendFormatter. Having the index enables quick indexing of external, unplotted auxiliary data for a point.

[coveralls]

Coverage increased (+0.02%) to 90.199% when pulling 6d639bcbe8e7d64594e894018b8e8e0439fd6755 on buck54321:legend-i into 27cee3f7233fbbbeed31f0cf03ea81bf92555b18 on danvk:master.