using Observables, Plots

angle = Observable(pi/2)

Observable{Float64} with 0 listeners. Value:
1.5707963267948966

function plot_arc(angle_value)
    angles = range(0, angle_value, length=100)
    x = cos.(angles)
    y = sin.(angles)
    plt = plot(
        x, y, 
        ratio=:equal, 
        xlims=(-1.5, 1.5), ylims=(-1.5, 1.5), 
        legend=:none, 
        framestyle=:none)
    display(plt)
end

plot_arc (generic function with 1 method)

angle_observer = on(plot_arc, angle)

(::Observables.ObserverFunction) (generic function with 0 methods)

notify!(angle)

angle[] = pi*2/3

2.0943951023931953