Why doesn't my chart pick up years as the horizontal axis?

Supposed to -- we would all like to hope so. In actual practice, I find that Excel is kind of hit and miss. I'm not exactly sure how Excel decides when to use data in rows vs. data in columns when creating a chart, or when to use 1st row/column for x data and when not to.
For myself, I find that I've started almost ignoring Excel's first guess when I create a chart. I create the chart, then end up going into the "select Data" dialog and removing and editing and adding data series until I end up with the chart I want.
I think if we spent enough time with it, we would find some tricks that will help Excel recognize when to use first row/column for x data. One I see is to make sure the upper left cell of the desired chart data range is blank when you create the chart. I don't know how reliably it works, but it might be worth considering.