DTVF: Plot multiple time series in the same graph

[sandradeng20]

It will be easier for data analysis if DTVF can plot multiple time series in the same graph.

[mdhillman]

I believe this should be technically possible within the visualisation, however there's some design that needs to be considered here.

Either, we add controls that allow any timeseries to be shown on top of any other timeseries, or we need to store some triples that define which timeseries are considered linked and will be shown together.

[mdhillman]

This is closely related to #659, a solution for one should fit the other. See the comments on that issue for more details.