More options for displaying multiple traces in a plot

Once a plot is created with multiple traces (e.g. with this: #160) it would be good if there were options in the menu to be able to view the plot in multiple ways (different datasets in these examples):

- Default:

- Stacked (optional parameter for spacing/scaling, e.g. >100% can be used in traces similar)

- Heatmap

- Simple Spike raster (optional parameter: threshold for spiking)

- Simple rate plots (i.e. spike freq generated from above; optional param: bin size)

Start by reviewing the multiple-trace plotting work referenced in issue #160 and the menu that appears after a plot is created. Define how the requested default, stacked, heatmap, spike-raster, and rate views should be selected and parameterized; done means each listed view works for multiple traces with its optional settings.

Written by the indexing model from the issue text.