See more discussion here: [url]TPad::Update() vs. TPad::Modified()
Basically when using the interpreter, pressing Enter makes the canvases update (as does clicking on a canvas). When running code non-interactively, the canvases only update when certain things happen (e.g. drawing something), so you will generally want to manually call the TPad::Update() method when doing graphical stuff in batch mode.
Jean-François