Unexplained gap in histogram

I'm creating a histogram in WPF using CategoryAxis and ColumnSeries. I've set the gap width to 0 and ensured the bar width is 1.0, but I'm still getting a gap between the columns. Curiously, when I run the histogram examples, there's no gap, but if I build a new chart using the same code, a gap appears. I traced into CategorySeries.GetRectangle(), and it appears to be producing the correct results - no gap between the rectangles - but that's not what's showing up on the screen. Any idea what's going on? Could this have something to do with WPF fudging the rectangles?

Yes, I guess this could be a "snap to device pixels" problem. Can you attach a screen shot?