A common way to show useful information on the canvas is to add a head up display, or graphical overlay to indicate particular values. I'm thinking about how to do gradients properly - the idea is to have a gradient tool that puts lines on each gradient representing the vector and the knot points. Clicking a knot point either pops up a dialog or a contextual menu or something to edit the colour at that point. There are plenty of other tools that could be added - things like control points for making dia type diagrams, e.g.
To do this I need to work out how to add stuff over the top of the canvas
(12:04:49) MenTaLguY: njh: adding canvas items to SPSVGView::parent
Well, the top group of canvas items is in SPSVGView::parent believe you use sp_canvas_item_new() to create a new item