add baseline to graphics

[huss]

Once baselining #209 is done, then we need to make it work with the graphics. This means as we aggregate into groups, we need to subtract the baseline from the value if desired by the user. This will be harder since the baseline can change over time and may not apply to the full graph being displayed.

Two ideas on how user can include: 1) an option on the graph; 2) create a group with only this meter that subtracts the baseline (special group type). I suspect choice 1 is what people want but we can check.

[huss]

There is now a design document for how this will be done that reflects a new design and the updated capabilities in OED.