ordinal must be >= 1 Error

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

I sometimes see this when a date graph is open in a GUI window and I
am moving my mouse. The mouse as is passes off the axes can return
invalid dates to the matplotlib date conversion routine. I think I
added some guards against this in the last release (0.86.2). What
version are you using.

In a nutshell, it likely means that somewhere in the pipeline
matplotlib is getting an illegal date value.

Next time you get the error, lease post version info and a complete