Draw the graph frame pixel-crisp

With cairo, if you want to draw a one-pixel-wide line, you need to put
the coordinates in the middle of the pixels, hence the -0.5 and +1.
New Graph option: graphBorderWidth

The main purpose of this is to be set to 0 to disable the border. But
who knows, maybe someone likes thicks borders...
Also make the vertical bar widget border pixel-exact