Overriding Default Grouping with the group Aesthetic

How Grouping Works in Lets-Plot-Kotlin

Default Grouping Behavior:

• Lets-Plot-Kotlin automatically groups data by discrete variables mapped to aesthetics like color, shape, linetype, etc.
• This creates separate visual elements (lines, paths, polygons) for each unique combination of these variables

Explicit Group Control:

• Use group = "var" to group only by that specific variable, overriding default grouping
• Use group = listOf(var1, var2, ...) to group by the interaction of multiple variables
• Use group = emptyList<String> to disable all grouping completely

%useLatestDescriptors
%use dataframe
%use lets-plot(output="js, png")

LetsPlot.getInfo()

Lets-Plot Kotlin API v.4.12.0. Frontend: Notebook with dynamically loaded JS. Lets-Plot JS v.4.8.1.
Outputs: Web (HTML+JS), Static PNG (hidden)

val df = DataFrame.readCSV("https://raw.githubusercontent.com/JetBrains/lets-plot-docs/refs/heads/master/data/mpg.csv")
val mtcars = df.toMap()
df.head()

DataFrame: rowsCount = 5, columnsCount = 12

untitled	manufacturer	model	displ	year	cyl	trans	drv	cty	hwy	fl	class
1	audi	a4	1,800000	1999	4	auto(l5)	f	18	29	p	compact
2	audi	a4	1,800000	1999	4	manual(m5)	f	21	29	p	compact
3	audi	a4	2,000000	2008	4	manual(m6)	f	20	31	p	compact
4	audi	a4	2,000000	2008	4	auto(av)	f	21	30	p	compact
5	audi	a4	2,800000	1999	6	auto(l5)	f	16	26	p	compact

1. Highway MPG by Drive Type

val seed = 21
letsPlot(mtcars) { x = "drv"; y = "hwy" } +
    geomViolin(tooltips = tooltipsNone) +
    geomSina(seed = seed)

2. Add More Information - color

letsPlot(mtcars) { x = "drv"; y = "hwy" } +
    geomViolin(tooltips = tooltipsNone) +
    geomSina(seed = seed) {
        color = "cyl"
    }

3. Discrete color: Default Grouping Creates Unwanted Separation

Let's add discrete colors by marking the cyl variable as discrete.

When we map color=asDiscrete("cyl"), Lets-Plot automatically groups the data by the discrete color variable.
This means:

• Automatic grouping: Each combination of drv (x-axis) and cyl (color) becomes a separate group
• Position adjustment: The geomSina() uses "dodge" positioning by default, which separates overlapping groups horizontally
• Result: Instead of one sina plot per drive type, we get 4 separate sina plots (one for each cylinder count) within each drive type category

letsPlot(mtcars) { x = "drv"; y = "hwy" } +
    geomViolin(tooltips = tooltipsNone) +
    geomSina(seed = seed) {
        color = asDiscrete("cyl")
    }

4. Fix with Explicit Grouping by Drive Type

letsPlot(mtcars) { x = "drv"; y = "hwy" } +
    geomViolin(tooltips = tooltipsNone) +
    geomSina(seed = seed) {
        color = asDiscrete("cyl")
        
        group = "drv"      // <-- group only by drive type (ignoring the color variable for grouping)
    }

5. Cleaner Fix: Disable All Grouping

letsPlot(mtcars) { x = "drv"; y = "hwy" } +
    geomViolin(tooltips = tooltipsNone) +
    geomSina(seed = seed) {
        color = asDiscrete("cyl")
        
        group = emptyList<String>()     // <-- disable all grouping entirely
    }