Julia 0.3

Julia 0.4

Gadfly is a plotting and data visualization system written in Julia.

It's influenced heavily by Leland Wilkinson's book The Grammar of Graphics and Hadley Wickham's refinment of that grammar in ggplot2.

It renders publication quality graphics to PNG, Postscript, PDF, SVG. The SVG backend uses embedded javascript, powered by Snap.svg to add interactivity like panning, zooming, and toggling.

Check out the manual for more details and examples.

Installation

From the Julia REPL a reasonably up to date version can be installed with

Pkg.add("Gadfly")

This will likely result in two dozen or so other packages also being installed.

Optional: cairo, pango, and fontconfig

Gadfly works best with the C libraries cairo, pango, and fontconfig installed. The PNG, PS, and PDF backends require cairo, but without it the SVG backends (SVG and SVGJS) are still available.

Complex layouts involving text are also somewhat more accurate when pango and fontconfig are available.

Julia's Cairo bindings can be installed with

Pkg.add("Cairo")

Three Invocations

All interaction with Gadfly is through the plot function, which takes three major forms.

Orthodox

plot(data::AbstractDataFrame, elements::Element...; mapping...)

This form is the standard "grammar of graphics" method of plotting. Data is supplied in the form of a dataframe, columns of the data are bound to aesthetics, and plot elements including scales, coordinates, statistics, guides, and geometries are added to the plot.

All of the examples that follow will be plotting data from RDatasets.

To render these plots to a file, call draw on the resulting plot.

draw(SVG("myplot.svg", 6inch, 3inch), plot(...))

A few examples now.

# E.g.
plot(dataset("datasets", "iris"),x="SepalLength", y="SepalWidth", Geom.point)

# E.g.
plot(dataset("car", "SLID"), x="Wages", color="Language", Geom.histogram)

A catalog of plot elements is given later in this document.

Heretical

plot(elements::Element...; mapping...)

Along with the orthodox invocation of plot, some relaxed invocations of the grammar exist as a "slang of graphics". This form of plot omits the the data frame. Instead, plain old arrays are bound to aesthetics.

# E.g.
plot(x=collect(1:100), y=sort(rand(100)))

If no geometry is specified, like in the example above, a Geom.point is stuck into your plot.

This plot otherwise works the same. We might want to name these axis, for example.

# E.g.
plot(x=collect(1:100), y=sort(rand(100)),
     Guide.XLabel("Index"), Guide.YLabel("Step"))

Functions and Expressions

plot(f::Function, a, b, elements::Element...)

plot(fs::Array, a, b, elements::Element...)

Some special forms of plot exist for quickly generating 2d plots of functions.

# E.g.
plot([sin, cos], 0, 25)

Elements

Plot elements in Gadfly are statistics, scales, geometries, and guides. Each operates on data bound to aesthetics, but in different ways.

Statistics

Statistics are functions taking as input one or more aesthetics, operating on those values, then output to one or more aesthetics. For example, drawing of boxplots typically uses the boxplot statistic (Stat.boxplot) that takes as input the x and y aesthetic, and outputs the middle, and upper and lower hinge, and upper and lower fence aesthetics.

Scales

Scales, similarly to statistics, apply a transformation to the original data, typically mapping one aesthetic to the same aesthetic, while retaining the original value. The Scale.x_log10 aesthetic maps the x aesthetic back to the x aesthetic after applying a log10 transformation, but keeps track of the original value so that data points are properly identified.

Geometries

Finally, geometries are responsible for actually doing the drawing. A geometry takes as input one or more aesthetics, and used data bound to these aesthetics to draw things. The Geom.point geometry draws points using the x and y aesthetics, the Geom.line geometry draws lines, and so on.

Guides

Very similar to geometries are guides, which draw graphics supporting the actual visualization, such as axis ticks and labels and color keys. The major distinction is that geometries always draw within the rectangular plot frame, while guides have some special layout considerations.

Drawing to backends

Gadfly plots can be rendered to number of formats. Without cairo, or any non-julia libraries, it can produce SVG. Installing cairo gives you access to the PNG, PDF, and PS backends. Rendering to a backend works the same for any of these.

some_plot = plot(x=[1,2,3], y=[4,5,6])
draw(PNG("myplot.png", 6inch, 3inch), some_plot)

Using the SVGJS backend

The SVGJS backend writes SVG with embedded javascript. There are a couple subtleties with using the output from this backend.

Drawing to the backend works like any other.

draw(SVGJS("mammals.js.svg", 6inch, 6inch), p)

If included with an <img> tag, it will display as a static SVG image.

<img src="mammals.js.svg"/>

For the interactive javascript features to be enabled, the output either needs to be included inline in the HTML page, or included with an object tag, like.

<object data="mammals.js.svg" type="image/svg+xml"></object>

Reporting Bugs

This is a new and fairly complex piece of software. Filing an issue to report a bug, counterintuitive behavior, or even to request a feature is extremely valuable in helping me prioritize what to work on, so don't hestitate.