Savefig() in Plots.jl is slow when I scatter 13056 points

General Usage
21

I wish this was just an issue only caused by large datasets. On Plots.jl version 1.20.0, just 10 scatter points and a straight line drawn based on the parameters returned by LsqFit.jl seems to be extremely slow, regardless of the output format:

 8.972615 seconds (20.30 M allocations: 1.104 GiB, 4.26% gc time, 2.74% compilation time)

The code that generates these numbers goes as follows:

function draw_scatter_and_line(
    xdata,
    ydata,
    gradient,
    constant_term,
    scatter_title,
    line_title,
    xlabel,
    ylabel,
    filename
)
    img_path = joinpath(@__DIR__, filename)
    
    # Generate plot object
    img = Plots.plot(
        xlabel=xlabel,
        ylabel=ylabel,
        legend=:bottomright)

    # Measurements
    Plots.scatter!(
        img,
        xdata,
        ydata,
        label=scatter_title,
        color=:steelblue)

    # Generate fit
    fit_x = [xdata[begin] - abs(xdata[end] / 10), xdata[end] + xdata[end] / 10)
    fit_y = gradient .* fit_x .+ constant_term

    # Draw fitted line:
    Plots.plot!(
        img,
        fit_x,
        fit_y,
        linestyle=:dash,
        label=line_title,
        color=:steelblue)

    # The bottleneck    
    @time Plots.pdf(img, img_path)
end

The compilation times of Plots.{pdf, savefig} seem irrelevant when looking at how many allocations they are doing. Again, xdata and ydata are vectors of length 10…

22

The allocations may also happen during compilation, they’re also counted towards the total allocation amount during the timed section.

23

I guess this is why it is generally recommended to use more robust benchmarking tools. Regardless, even during a second run there are roughly 250k allocations made by Plots.savefig alone, in the case of the mentioned 10 data points.