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Adding layers in Gadfly (Julia)

It's not really very clear from the documentation of Gadfly package how to add layers to an existing plot outside of the first plot statement, or how to display a plot once a layer has been added. After asking around, I finally figured out how to do it:

# tested in IJulia notebook

using Gadfly
# the package is slow the first time you load it AND the first time you plot

x = [-π:0.1:π]
y = sin(x)
w = cos(x)

pi = round(π, 2)
ticks = [-pi:pi/2:pi]

p = plot(x=x, y=y, Geom.point, Geom.line, Theme(default_color=color("seagreen")),
            Guide.xticks(ticks=ticks))
q = layer(x=x, y=w, Geom.line, Theme(default_color=color("indianred")))
append!(p.layers, q);

# draw(SVGJS("sin-cos.svg", 7.5inch, 3.25inch), p)
display(p)    # works as a side effect! (unlike `plot(...)` or just `p`)
2+2

The commented-out draw statement will save the image in an SVG format (feel free to interact with the Gadfly plot below).

To embed it above in this post, I entered:

<object type="image/svg+xml" data="\images\sin-cos.svg"></object>
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