problem with "fuzzy" grid lines


I use matplotlib to non-interactively convert a pile of numbers into a
diagram for may web page. My problem: the resulting grid lines are
not perfectly straight, they seem to be undecided which of two pixel
rows they belong to. The lines (magnified and in ASCII art) look
a bit like

                            ******** ******** ****
    ******** ******** ****** ****

How can I get straight and homogeneous lines?

The gory details:

(1) This is a self-compiled version of matplotlib-0.86.2 on a Debian
  Sarge system. I used the following options in

    rc = dict({'backend':'Agg', 'numerix':'numarray'})
    BUILD_AGG = 1
    BUILD_GTK = 0

(2) The following code was used to generate the picture:

    import matplotlib
    from pylab import *
    from matplotlib.dates import DayLocator, WeekdayLocator, DateFormatter, \

    ...prepare the data...


    axes([0.07, 0.58, 0.89, 0.36])
    ax = gca()
    fill(k+k[::-1], xv+xs[::-1], fc="g", ec="g")
    fill(k+[k[-1],k[0]], xs+[0,0], fc="r", ec="r")

    axes([0.07, 0.20, 0.89, 0.36])
    ax = gca()
    fill(k+k[::-1], yv+ys[::-1], fc="g", ec="g")
    fill(k+[k[-1],k[0]], ys+[0,0], fc="r", ec="r")

    labels = ax.get_xticklabels()
    setp(labels, rotation=90)


(3) You can see the resulting picture at

  The problem seems to occur at every second horizontal grid line in
  the lower panel.

Many thanks,