Bizarre dates in X-axis of chart

I’m trying to create a XY (Scatter) chart from two simple columns of data. Column A is formatted as dates. The values I have input are being interpreted as dates and not text. When I change the cell formatting, the dates all reformat as expected.

Column A is supposed to be the X values for my chart. The chart’s range for X values defaulted to $Sheet1.$A$1:$A$11, which seems to be correct. But the X values on the chart have no obvious relationship to the data. They’re not even in a sensible order. The first three values in the cells are 12/24/13, 12/31/13, and 1/12/14. The first three values in the chart are 9/6/92, 4/2/57, and 10/27/04!

What the heck is going on?

I discovered what was happening, but I don’t know how it happened. My date column was formatted M/D/YY. In one cell, I entered the date 2/1/14. Somehow that was interpreted as 2/1/214 instead of 2/1/2014.