Currently it's almost impossible to view tooltips for low value columns. I'd suggest to trigger the tooltip through a transparent (#fff0) rect below each column that spans the max height. You can then set the actual bar to pointer-events:none.

Good suggestion! I've created bars with the height equal to the max yScale, making it easier to select. Also added an option to highlight that background rect element, which gives a nice effect of being able to see how the bar stacks up compared to the max value.