What's the difference between a bar chart and a histogram?
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I disagree with wiki answers, which although it's partially correct, completely misses the main point. The heights in a bar chart represent counts. You can use a bar chart for this purpose with categorical data or with binned "continuous" variates; either is fine. The areas in a histogram represent relative frequencies or proportions per unit value of a continuous variate. A histogram thus is a discrete approximation to a frequency or probability density function, whereas a bar chart has no such meaning (except accidentally when it's really serving as a histogram!). The distinction comes to the fore when the bars in a histogram have varying widths.
Reference: Freedman, Pisani, Purves, Statistics. (Any of the first through fifth editions should be fine.)
With bar charts, each column represents a group defined by a categorical variable; and with histograms, each column represents a group defined by a quantitative variable.