See my full comments in the JIRA: https://issues.apache.org/jira/browse/CLIMATE-934
More or less what is happening is the first date in the series is 01-01-01. When figuring out the xticks / xtick labels for the time series, matplotlib is subtracting some value from that date to calculate the view min, but Epoch minus something is negative which results in trying to convert a negative number to a date and the error is thrown. If I update the example to shift the dates to the middle of the month it works without error. I could also change the utilscalc_climatology_monthly to return 01 - month - mid point of month day which would be neater. I could add an optional parameter to use the first, mid or last day of the month to avoid breaking anyone who expects it to be the first of the month. I could also dig into the plotter.py and see if there is a switch to tell it to apply the first xtick at 0 on the x rather than trying to calculate the offset. Feedback appreciated on the preferred approach.
