There's no time axis as this isn't particularly a time-based graph
The graph pretends to be a time-based graph, though, by highlighting the differences between the Bush and Obama administrations. It's not like it would take any more work or space to lay the information out horizontally. I'm sure I've seen it done before, but this is the closest I can find: http://en.wikipedia.org/wiki/File:US_Federal_Debt_as_Percent...