TODO for ZedGraph:

Stick item for stick charts, sticks go to X axis (or base axis)
Expose Line Style for ArrowItem, StickItem

Multiple Y axes (add Y3, Y4, etc.)

PolyItem for polygons, not necessarily closed.

Add a drawing tool to the usercontrol that lets you add polyitems via the mouse.
Also add the capability to select items for deletion, etc.


ToolTips:
When I have 'show point values' turned on for a graph with multiple
line items point values are shown whether a line item is visible or
not. I guess the logic does not check whether the line item is visible
hence a minor issue.

Publish/Subscribe:
The bigger things I am looking forward to is some publish/subsribe
mechanism (to notify interested parties of events taking place such as
zoom/unzoom, pan/unpan, x/y min/max changed, scroll bars (to stabilize
the panning).

crosshairs:
Another feature would be a cross of a horizontal and vertical line
rendered in some color from x/y min/max respectively. The cross would
be movable with the mouse that drags the point where the lines cross.
It would then provide the coordinates of the cross preferable by means
of a notification as well as a 'show point value' like way. Such a
cross could be used to know the location and values of items on that
location if any. Another use for such a cross is for defining a
quadrant cut to zoom to or to create a rectangle by selecting two
locations in this manner, the latter is an alternative way of creating
a zoom rectangle with the mouse as currently exists and a allows for a
bit more precision).
Sometimes these lines are called 'hair lines'. With two of these lines
you can select a first point in 2d space and after that select another
point to create a rectangle. Sometimes a single point defined this way
is all that's needed, sometimes two.

Extensible menu:
And finally, are the popup menu's extensible, i.e. when right-clicking
on a graph a menu pops up, can I add my own commands to that menu?

Scrollbars:
Given the recently added panning feature, a logical addition to the control would be to use scroll bars as an alternative to panning by dragging (but dragging and scrolling would keep one another in sync of course so alternative does not equal an exclusive or). Scroll bars would give a clue as to the position (where does the thumb start?) and the relative amount (the width of the thumb) of the view shown (the view being defined as per example the subrange [1067..7528] of range [0..1000000]).
Just a thought.

More Symbol Types:

Line Segment coloration by X/Y/Z