Error bars in histograms remind me of detonators used to set off explosives. I am tempted to ask: "What would happen if you pushed down the plunger?" You see, in the old days people used to show the whole error bar, not just the half sticking out of the top. I'm still getting used to them, but I'm not always sure what they mean. Turns out that I'm not alone. Cummings et al. wrote a paper illustrating the ways (several) in which the same innocent-looking gizmo can be used. They conclude with "eight simple rules to assist with effective use and interpretation of error bars." Have a look.