[Matplotlib-users] ordinal must be >= 1 Error

[Panos K. <pk...@in...>]

Hello all,

I am using matplotlib for a web application to create trend charts 
from measurements stored in a database.
Specifically I use plot_date to plot the values vs their timestamps.
Occasionally and without any obvious to me reason, plot_date raises 
the exception

"ordinal must be >= 1"

I check that both the x and y value sequences are not empty and their 
length is certainly above  1.
Does anybody know where this "ordinal" refers to and what might be 
causing this error?
Thank you very much

				Panos