#507 Make it easier to highlight multiple observers on a plot

[David Benn]

If multiple filters were allowed to be applied at the same time with different colors, the step of creating new series could be avoided. So, two things need to be done, and this would be generally useful for all filters, not just by observer code:

1. Allow a filter to have a different color.
2. Allow multiple filters to be applied at the same time.

Probably the easiest way to do this is to allow a new series to be created immediately at the time of filter creation. If the filter is named, we could automatically go into series-creation mode.