Boxplot is probably one of the most common type of graphic. It gives a nice summary of one or several numeric variables. The line that divides the box into 2 parts represents the median
of the data. The end of the box shows the upper and lower quartiles. The extreme lines shows the highest and lowest value excluding outliers. Note that boxplot hide the number of values
existing behind the variable. Thus, it is highly advised to print the number of observation, add unique observation with jitter or use a violinplot if you have many observations.