BoxLineStyle

is an option for BoxWhiskerPlot that specifies styles to be applied to the lines drawn in the plot.

Details and Options

To use BoxLineStyle, you first need to load the Statistical Plots Package using Needs["StatisticalPlots`"].
Valid settings are the same as valid graphics directives for lines.
A list of graphics directives can be given for multiple box-and-whisker plots, in which case directives are applied cyclically to plots.

Use dashed rather than solid lines in the plot:

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