ordinal must be >= 1 Error

Panos, Ordinal in this context is the y values. It's not the

    > length of the y array, it's the values in the array. Dates
    > are days past some epoch and dates before the epoch are not
    > allowed. Maybe you're getting a date value that's < 1 which
    > is what is causing the problem. Are you plotting dates on
    > the y axis anywhere?

Here ordinal is used in the sense of the python datetime toordinal
method, not in the ordinal / abscissa of the y / x axis. Admittendly
ambiguous. But I think your first comment is spot on: probably an
illegal ordinal value is used, eg

  import datetime
  print datetime.datetime.fromordinal(1)
  print datetime.datetime.fromordinal(0)