For plot marking:
=================

- use Alt-Left Click to created a plot marker at the current position
- use a vertical dotted foreground-colour line to mark the plot.
- all other zooming/data mode actions are still available.

Add to Plot Dialog
------------------
- plot markers will be part of plots, not curves.
- have a new tab on the Plot Dialog called Markers
- on the tab will be a list of plot markers
  (probably in x-coordinate form)
- underneath/beside the list of plot markers will be 3 buttons:
    - Add (to manually add a plot marker by entering the value)
    - Remove (this will remove selected plot marker(s))
    - Remove All

Add to Plot Context Menu
------------------------
- add a separater to the bottom of the Scroll submenu in the context menu
- underneath the separater, add two entries:
    - Next Plot Marker
    - Previous Plot Marker
- suggested shortcuts - Alt+Right arrow/Alt+Left arrow, PgUp/PgDown,...
- moving to plot markers is a scrolling action, it will not change the
  zoom "level"

Additional Details
------------------
- there will be public functions for adding, removing, and retrieving plot
  markers from plots (event monitors could use these to add markers,
  given that a plot is specified to be associated with the event).  



