Your comments

I was using Background="{0:null}" and not Background="Transparent" (WPF) and was not getting zoom etc to work - now I know why.

With a null background, mouse/gesture events (such as zoom), are not captured - I have to have the cursor over a drawn element such as series line or the numbers of the axis for zoom to work when there is a transparent background. Using Transparent fixed it. null backgrounds sound useful in some situations though when you want background controls to get mouse events - good to know!