Boxplots are useful visualization tools for see the average and spread of data. The box shows the interquartile range (middle 50% of the data) and the median is marked as the center line. The whiskers show the outer edge of the distribution (although the definitions vary). Here we are showing 1.5 times the interquartile range. Any outliers beyond that are shown individually.
Although boxplots are great in their simplicity, they hide the details of the inner distribution of the data. In the demonstration below, you may change the mean and spread of the data, and the number of samples. See how the boxplot responds. Then, move any datapoints by clicking and dragging.
There are particular pathological cases that have the same boxplot, but very different distributions. You can see more about this here. You should also check out Anscombe's Quartet for why you should always plot your data.
Use sliders below to generate random gaussian data. Perturb individual datapoints by dragging them.
D3 boxplot from: Mike Bostock