Seaborn's boxplot whiskers meaning

Question

I'm working with seaborn's box plot, and I can't seem to figure out the placement of the whiskers in the default settings.

Looking at the seaborn.boxplot docs I see that whis=1.5 which I assumed means that the whiskers are placed at UPPER_QUARTILE + IQR*1.5 and at LOWER_QUARTILE - IQR*1.5.

But even in the docs themselves, looking at the whiskers, we can see that they have different gap lengths from the upper/lower quartiles

It is easy to see that the lengths on both sides of the box to the whiskers are not equal, so my assumption is obviously wrong.

So how the default whiskers are placed? Or maybe I fail to understand something more basic about the nature of the box plot?

Answer 1

IIRC, the whiskers extend to the the lowest (highest) data point still within 1.5 IQR of the lower (upper) quartile. So depending on where the data points actually are, the whiskers on both sides won't necessarily be of the same length.

Matplotlib docs for the whis argument of boxplot() (which Seaborn is built on top of) seem to confirm this:

whis : float, sequence, or string (default = 1.5) As a float, determines the reach of the whiskers to the beyond the first and third quartiles. In other words, where IQR is the interquartile range (Q3-Q1), the upper whisker will extend to last datum less than Q3 + whis*IQR). Similarly, the lower whisker will extend to the first datum greater than Q1 - whis*IQR. Beyond the whiskers, data are considered outliers and are plotted as individual points.