How do you mark an outlier on a box plot?

With a cross. This will be outside the lines that represent the maximum and minimum

With a cross to denote the outliers.

Outliers for box plots are defined by being more than 1.5*IQR (Interquartile Range) under the first quartile (Q1), or more than 1.5*IQR (Interquartile Range) over the third quartile (Q3). They are usually indicated as a cross or dot, outside of the box plot and unconnected to it.

Outliers are usually identified on a box plot by a cross.

You mark an outlier using an 'x' or a '+'. An outlier is defined as a data point that lies outside the whiskers of the box plot, usually taken to be outside 1.5 times the interquartile range above the upper quartile and below the lower quartile.