zoom control not obvious

[rade]

@lukemarsden watched a user struggling to work out how to zoom in scope. They used the browser zoom, which only zooms everything except the graph. And they didn't realise that mousewheel zoomed at all.

I wonder whether a) browser zoom should zoom everything, and) we should add some google maps style zoom control in the bottom right corner.

[fbarl]

+1 for google maps style zoom controls

Browser zooming also zooms into the graph on my Chrome, but I don't think we should have people think that's the right way to do graph zooming :)