Bar charts summary

stat

stat can be "identity" (we specify the value of the y manually, like in a scatterploy) or "count" (in which case the special values ..count.. and ..prop.. are computed and can be mapped to the y axis). bin is another option, but we use geom_histogram for that.

position

position can be "identity", in which case the bars are displayed according to the x value without modification (overlayed on each other as necessary), "dodge", in which bars that would otherwise overlap are displayed side-by side (“dodging” each other), or "stack", in which case bars with the same x value are stacked on top of each other. When using position = "identity" it often makes sense to specify alpha = 0.5 (or something like that) to make the bars semi-transparent.

group

See Healy Ch. 4.4

geom_histogram

geom_histogram is just geom_bar with stat = "bin". All the different position values discussed above can be used.