21 Polar Coordinates

For polar coordinates, (pj/coord :polar) maps x to angle and y to radius. Bars become arc-interpolated wedges (rose charts), and scatter points wrap onto a polar plane. The angular spread is set by the x-column’s range, so a narrow range produces a wedge cluster rather than a full circle. Supported marks today are :point, :bar, :rect, :text, and :rug; other marks raise a clear error. Tick labels and axis labels are not yet rendered under :polar, but legends render as they would on Cartesian coords – a :color mapping produces a normal legend that can be repositioned via :legend-position (see Customization).

(ns plotje-book.polar
  (:require
   ;; Rdatasets -- standard datasets
   [scicloj.metamorph.ml.rdatasets :as rdatasets]
   ;; Kindly -- notebook rendering protocol
   [scicloj.kindly.v4.kind :as kind]
   ;; Plotje -- composable plotting
   [scicloj.plotje.api :as pj]))

(def wind {:direction ["N" "NE" "E" "SE" "S" "SW" "W" "NW"]
           :speed [12 8 15 10 7 13 9 11]})

Polar Scatter

The same scatter plot, wrapped onto a polar plane. x maps to angle (clockwise from 12 o’clock), y maps to radius (center = minimum, edge = maximum). Iris’s :sepal-length spans roughly 4.3 to 7.9 – a narrow numeric range – so the points cluster in an arc rather than spreading across the full circle.

(-> (rdatasets/datasets-iris)
    (pj/lay-point :sepal-length :sepal-width {:color :species})
    (pj/coord :polar))

Rose Chart (Coxcomb Diagram)

A bar chart in polar coordinates produces a rose chart – wedge area encodes count. Bars are arc-interpolated for smooth curved edges.

(-> (rdatasets/datasets-iris)
    (pj/lay-bar :species)
    (pj/coord :polar))

Wind Rose

A value-bar chart in polar makes a wind rose – each direction gets a wedge proportional to wind speed.

(-> wind
    (pj/lay-value-bar :direction :speed)
    (pj/coord :polar))

Polar Stacked Bar

Stacked bars in polar show composition within each wedge. Without angular tick labels (not yet rendered under :polar), the legend carries the category identity – read color back to species via the legend rather than around the wedge.

(-> (rdatasets/palmerpenguins-penguins)
    (pj/lay-bar :island {:position :stack :color :species})
    (pj/coord :polar))

Polar Histogram

A histogram in polar wraps bins around the circle. Useful for circular distributions (e.g., time of day, compass bearing).

(-> (rdatasets/datasets-iris)
    (pj/lay-histogram :sepal-length)
    (pj/coord :polar))

Explicit Labels with Polar

By default, polar suppresses auto-generated axis labels (since there are no rectangular axes). You can still set a title with pj/options:

(-> (rdatasets/datasets-iris)
    (pj/lay-point :sepal-length :sepal-width {:color :species})
    (pj/coord :polar)
    (pj/options {:title "Iris in Polar Space"}))

What’s Next

Cookbook – recipes for common multi-layer plots
Customization – colors, annotations, and interactive features

source: notebooks/plotje_book/polar.clj