uneven width of bars in axes.bar()


I’ve started to use matplotlib for my website since
about a month ago and so far I am very impressed with it. I have one
question about bar plots using the axes.bar() function. It seems that
sometimes when I specify a constant width (by supplying a scalar width
argument), I get slightly uneven bar widths. Moreover, dending on the
width I specify and the limits of the x-axis, some bars get dropped
altogether. I am using the savefig() function to save the plots as a PNG
file. An example of uneven bar widths can be seen here:

I would appreciate it if anyone has
encountered this problem before and if there is a fix for it. Thank