Adding text and rectangles below plotted data?

I would like to be able to draw rectangles, with text centered inside
them, at key positions on a plot.

The case in question is a bar plot, and the rectangles need to be drawn
such that they are length of one or more of the each bar groupings,
side-by-side in a line along the width of the X-axis. The line of
rectangles should be located just below the X-axis tickmark labels.

I see the fill() command allows plotting of a polygon and the figtext()
command allows text to be plotted so, in theory, these should work.

My problem is

(a) what is the syntax for the fill() command - I have tried
ax.fill(0.5,0.5,'b') and get an error:
  TypeError: zip argument #1 must support iteration

(b) how to calculate the correct (x,y) values for locating the
start/stop points of the rectangles? i.e. how do I find out where the
start/end points are for the X values of each bar grouping? And at what
vertical height should the line of rectangles be located?

Any ideas or insights would be appreciated?



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