Order of arguments in plotting

New to Julia
,
1

This is an example from QuanEcon Section 31. Optimal Growth I.

The following code tries to plot the linear interpolation and the true value of a function f.

f(x) = 2 .* cos.(6x) .+ sin.(14x) .+ 2.5
c_grid = 0:.2:1
f_grid = range(0,  1, length = 150)

Af = LinearInterpolation(c_grid, f(c_grid))

plt = plot(xlim = (0,1), ylim = (0,6))
plot!(plt, f, f_grid, color = :blue, lw = 2, alpha = 0.8, label = "true function")
plot!(plt, f_grid, Af.(f_grid), color = :green, lw = 2, alpha = 0.8,
      label = "linear approximation")
plot!(plt, f, c_grid, seriestype = :sticks, linestyle = :dash, linewidth = 2, alpha = 0.5,
      label = "")
plot!(plt, legend = :top)

And the graph will be as follows

Screen Shot 2021-12-24 at 15.28.07
Screen Shot 2021-12-24 at 15.28.071246×830 46.5 KB

However, if I change from

plot!(plt, f_grid, Af.(f_grid), color = :green, lw = 2, alpha = 0.8,
      label = "linear approximation")

to

plot!(plt, Af.(f_grid), f_grid, color = :green, lw = 2, alpha = 0.8,
      label = "linear approximation")

It will give me a different graph.

But, I have found that the graph doesn’t change if I change the order of arguments of f and f_grid. i,e

plot!(plt, f, f_grid, color = :blue, lw = 2, alpha = 0.8, label = "true function")

will give us the same graph as

plot!(plt, f_grid, f,color = :blue, lw = 2, alpha = 0.8, label = "true function")

I would appreciate it if someone can help me with this issue. To be more specific, why change of order in some command will result in a different graph while change the order in some commands will lead to the same graph.

2

(don’t forget to mention any packages you use, I’m assuming you’re plotting with Plots.jl)

Normally, plots assumes that arguments are in order y, x, y or x, y, z, but it also uses dispatch. It has, for instance, been decided that if the first argument isa Function, the next is rather a domain than a y, because it seems unlikely you would want to plot a y vector vs an x function.

Essentially, the fact that transposing doesn’t matter in one of those cases is the exception.

1 Like
3

Currently plot(x, g) == plot(g, x) == plot(x, g.(x)), but it would make sense to have: plot(g⁻Âč, y) to mean plot(g⁻Âč.(y), y)

For example:

using Plots; gr(legend=false, lw=3)
g(x) = log(x)
g⁻Âč(y) = exp(y)
x = 0.1:0.01:2
plot(x, g)
y = g.(x)
# plot!(g⁻Âč, y)  # this could be interpreted instead as: plot!(g⁻Âč.(y),y)
plot!(g⁻Âč.(y), y, ls=:dash)
1 Like
4

I think that’s rare enough it makes sense to force you to be explicit. But it’s a somewhat agree for me.

It definitely won’t change though.