Add option to specify data set (line) caption when creating new diagram

[ghorwin]

When creating a variant study diagram, using different tsv files (or reference-format DataIOs), for example.

NANDRAD.tsv
DELPHIN.tsv
THERAKLES.tsv

and using data column 2 "TemperatureA" from each file,
the generated lines have the caption "$quantity" which shows for all 3 lines only "TemperatureA". It would be more meaningful to use "$basename" here.

Since this is not always wanted, it might be useful to offer a customization, just as the mapper title in the same line.