Setting a bar graphs x axis width?

Hello,

I’m trying to create a line graph that plots some values (y) against time (x).
There are a few possibilities I want to investigate, both involving toying with the x axis.

The main thing I’d like to do is set a dual label along the x axis. An example here:
http://i.stack.imgur.com/XBDhQ.png

My own preference would be to have days and weeks split into two.
And then, to take the further, possibly plot a secondary bar graph from the weeks cumulative value, with the line graph representing day by day data.

Any Help?