Change plot style when in stacked plot mode, such that
plots stack as expected. (Without this change, the
gl-plot class's height of 100% will cause all plots to
fill the full vertical space, effectively acting
overlaid.) WTD-625.
Begin refactoring Plot view to support stacked plots;
separate out behavior which needs to occur per-plot
from area which should occur per-view, and add a
handler for the Overlay mode. WTD-625.
Get/set mode from the plot mode switcher; only show
the switcher if there is more than one mode available
(which should only occur when there is more than one
telemetry object in the plot.) WTD-625.
