polar

Transforms Cartesian coordinates into a polar coordinate system. The x-axis maps to angle (theta) and the y-axis maps to radius.

from gofish import chart, stack, rect, polar

chart(seafood, coord=polar()) \
    .flow(stack(by="species", dir="x")) \
    .mark(rect(w="count", fill="species")) \
    .render(w=400, h=300)

Signature

polar(
    inner_radius: float | None = None,   # donut hole, fraction [0,1) of outer radius
    central_angle: float | None = None,  # total sweep in radians (default 2π)
    start_angle: float | None = None,    # angle (radians) of θ=0 (default π/2)
    direction: int | None = None,        # +1 CCW, -1 CW (default -1)
    center: tuple[float, float] | None = None,  # screen-space center offset
) -> Coord

Parameters

All optional; the defaults reproduce a centered, full-circle disc starting at 12 o'clock and going clockwise.

Option	Default	Description
inner_radius	0	Donut hole as a fraction [0,1) of the outer radius.
central_angle	2π	Total angular sweep in radians (use <2π for a partial fan).
start_angle	π/2	Angle (radians) where θ=0 sits (π/2 = 12 o'clock).
direction	-1	+1 counter-clockwise, -1 clockwise.
center	[0, 0]	Screen-space center offset.

Axis aliases

Inside a polar coord, dimensions can be named by their polar axis: theta (= x, angular position) and r (= y, radius), with extents thetaSize (= w) and rSize (= h). Like emX/emY, these mark options are camelCase. They coexist with x/y and are scope-bounded — valid only inside a coord that declares them. The operator dir accepts the angular/radial aliases too.

chart(data, coord=polar()) \
    .flow(spread(by="category", dir="theta")) \
    .mark(rect(thetaSize=0.4, rSize="value", emX=True, emY=True))

Usage

Pass the coordinate transform to chart via the coord keyword:

chart(data, coord=polar()) \
    .flow(...) \
    .mark(...) \
    .render(w=400, h=300)

coord may also be passed as a positional options dict — chart(data, {"coord": polar()}) — but the keyword form above is preferred in Python.

Coordinate Mapping

Cartesian	Polar
x	angle (theta), 0 to 2π
y	radius from center

Examples

# Basic polar chart
chart(data, coord=polar()) \
    .flow(stack(by="category", dir="x")) \
    .mark(rect(w="value"))

# Polar with spread for radial segments
chart(data, coord=polar()) \
    .flow(spread(by="month", dir="x")) \
    .mark(rect(w=1, h="value"))

# Donut: a hollow center (inner radius = 50% of the outer radius)
chart(data, coord=polar(inner_radius=0.5)) \
    .flow(stack(by="category", dir="x")) \
    .mark(rect(w="value"))

# Partial fan: a 270° sweep instead of the full circle
import math
chart(data, coord=polar(central_angle=3 * math.pi / 2)) \
    .flow(spread(by="month", dir="x")) \
    .mark(rect(w=1, h="value"))

See Also

• clock — Similar to polar but with 0° at 12 o'clock