comparing

comparing p x y = compare (p x) (p y)
Useful combinator for use in conjunction with the xxxBy family of functions from Data.List, for example:
... sortBy (comparing fst) ...
Lazily compare the length of two Foldables. > comparingLength [1,2,3] [False] == GT > comparingLength [1,2] "ab" == EQ > (xs :: [Int]) (ys :: [Int]) -> comparingLength xs ys == Data.Ord.comparing length xs ys > comparingLength 1,2 == LT > comparingLength (1:2:3:undefined) [1,2] == GT
a compare function that ignores the free "file" type variable:
Like Data.Ord.comparing. Helpful in conjunction with the xxxBy family of functions from Data.List
Here I combine errorBox with smallMultiples2 so we can compare the distribution (from the histogram) with that from the box plot. Open this visualization in the Vega Editor
let histEnc = encoding
. position X [ PName "Gmag", PmType Quantitative, PBin [] ]
. position Y yAxis
. color [ MName "Cluster", MmType Nominal, MLegend [] ]

errEnc = encoding
. position X [ PName "Gmag", PmType Quantitative ]
. position Y [ PNumber 80 ]
. color [ MName "Cluster", MmType Nominal, MLegend [] ]

yAxis = [ PAggregate Count
, PmType Quantitative
, PAxis [ AxTitle "Number of Stars" ]
]

boxOpts = [ MMedian [ MColor "black" ]
, MBox [ MStroke "white" ]
, MNoOutliers
]

histSpec = asSpec [ mark Bar [], histEnc [] ]
errSpec = asSpec [ mark Boxplot boxOpts, errEnc [] ]

combinedSpec = asSpec [ layer [ histSpec, errSpec ] ]

in toVegaLite
[ gaiaData
, columns 3
, facetFlow [ FName "Cluster", FmType Nominal ]
, specification combinedSpec
]
The main additions here are the configuration of the box plot - with MMedian, MBox (used to ensure the box is visually distinct from the bar for the Pleiades cluster, where they overlap), and MNoOutliers (to turn off the display of the outliers) - and the use of PNumber to define the location on the y axis of the boxplot visualization. Note that PNumber is defined in pixel units, with 0 being the top of the visualization and 80 was found by trial and error.
.thenComparing method.