Scatter plots with date axes

With the following snippet, I expect a vertical line from y=(0, 2) and
squares at y=(4, 5, 6) at the specified time. No squares appear with
the call to scatter, even though the y axis limits adjust to (0,7) as
if something is being plotted.

Is this a known limitation of scatter? I'm running the following under
ipython -pylab.

from datetime import datetime
f=figure()
ax=f.add_subplot(111)
d=datetime(2004,05,26,23,00,00)
d=date2num(d)
ax.xaxis_date()
ax.plot((d,d,d),range(3)) # vertical line
ax.scatter((d,d,d),(4,5,6),marker='s') # no symbols plotted
draw()

Thanks,
Eric