Gadfly is a system for plotting and visualization based largely on Hadley Wickhams's ggplot2 for R, and Leland Wilkinson's book The Grammar of Graphics.
From the Julia REPL a reasonably up to date version can be installed with
This will likely result in half a dozen or so other packages also being installed.
Gadfly is then loaded with.
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
Most interaction with Gadfly is through the plot function. Plots are described by binding data to aesthetics, and specifying a number of plot elements including scales, coordinates, guides, and geometries. Aesthetics are a set of special named variables that are mapped to plot geometry. How this mapping occurs is defined by the plot elements.
This "grammar of graphics" approach tries to avoid arcane incantations and special cases, instead approaching the problem as if one were drawing a wiring diagram: data is connected to aesthetics, which act as input leads, and elements, each self-contained with well-defined inputs and outputs, are connected and combined to produce the desired result.
If no plot elements are defined, point geometry is added by default. The point geometry takes as input the x and y aesthetics. So all that's needed to draw a scatterplot is to bind x and y.
Multiple elements can use the same aesthetics to produce different output. Here the point and line geometries act on the same data and their results are layered.
plot(x=rand(10), y=rand(10), Geom.point, Geom.line)
More complex plots can be produced by combining elements.
To generate an image file from a plot, use the draw function. Gadfly supports a number of drawing backends. Each is used similarly.
# define a plot
myplot = plot(..)
# draw on every available backend
draw(SVG("myplot.svg", 4inch, 3inch), myplot)
draw(SVGJS("myplot.svg", 4inch, 3inch), myplot)
draw(PNG("myplot.png", 4inch, 3inch), myplot)
draw(PDF("myplot.pdf", 4inch, 3inch), myplot)
draw(PS("myplot.ps", 4inch, 3inch), myplot)
draw(PGF("myplot.tex", 4inch, 3inch), myplot)
If used from IJulia, the output of plot will be shown automatically.
Plotting data frames
The DataFrames package provides a powerful means of representing and manipulating tabular data. They can be used directly in Gadfly to make more complex plots simpler and easier to generate.
In this form of plot, a data frame is passed to as the first argument, and columns of the data frame are bound to aesthetics by name or index.
# Signature for the plot applied to a data frames.
plot(data::AbstractDataFrame, elements::Element...; mapping...)
The RDatasets package collects example data sets from R packages. We'll use that here to generate some example plots on realistic data sets. An example data set is loaded into a data frame usinge the data function.
plot(dataset("datasets", "iris"), x="SepalLength", y="SepalWidth", Geom.point)
plot(dataset("car", "SLID"), x="Wages", color="Language", Geom.histogram)
Along with less typing, using data frames to generate plots allows the axis and guide labels to be set automatically.
Functions and Expressions
Along with the standard plot function, Gadfly has some special forms to make plotting functions and expressions more convenient.
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.