Plotting

Plotting

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.

Plotting arrays

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.

# E.g.
plot(x=rand(10), y=rand(10))
x 0.00 0.25 0.50 0.75 1.00 0.0 0.5 1.0 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.

# E.g.
plot(x=rand(10), y=rand(10), Geom.point, Geom.line)
x 0.0 0.5 1.0 0.0 0.2 0.4 0.6 0.8 1.0 y

More complex plots can be produced by combining elements.

# E.g.
plot(x=1:10, y=2.^rand(10),
     Scale.y_sqrt, Geom.point, Geom.smooth,
     Guide.xlabel("Stimulus"), Guide.ylabel("Response"), Guide.title("Dog Training"))
Stimulus 0.0 2.5 5.0 7.5 10.0 1.02 1.12 1.22 1.32 1.42 Response Dog Training

To generate an image file from a plot, use the draw function. Gadfly supports a number of drawing Backends.

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 using the dataset function.

using RDatasets
# E.g.
plot(dataset("datasets", "iris"), x="SepalLength", y="SepalWidth", Geom.point)
SepalLength 4 5 6 7 8 2.0 2.5 3.0 3.5 4.0 4.5 SepalWidth
# E.g.
plot(dataset("car", "SLID"), x="Wages", color="Language", Geom.histogram)
Wages 0 10 20 30 40 50 English Other French Language 0 50 100 150 200

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.

# E.g.
plot([sin, cos], 0, 25)
x 0 5 10 15 20 25 f1 f2 Color -1.0 -0.5 0.0 0.5 1.0 f(x)

Plotting wide-formatted data

Gadfly is designed to plot data is so-called "long form", in which data that is of the same type, or measuring the same quantity, are stored in a single column, and any factors or groups are specified by additional columns. This is how data is typically stored in a database.

Sometimes data tables are organized by grouping values of the same type into multiple columns, with a column name used to distinguish the grouping. We refer to this as "wide form" data.

To illustrate the difference consider some historical London birth rate data.

births = RDatasets.dataset("HistData", "Arbuthnot")[[:Year, :Males, :Females]]
RowYearMalesFemales
1162952184683
2163048584457
3163144224102
4163249944590
5163351584839
6163450354820

This table is wide form because "Males" and "Females" are two columns both measuring number of births. Wide form data can always be transformed to long form, e.g. with the stack function in DataFrames, but this can be inconvenient, especially if the data is not already in a DataFrame.

stack(births, [:Males, :Females])
RowvariablevalueYear
1Males52181629
2Males48581630
3Males44221631
............
162Females76231708
163Females73801709
164Females72881710

The resulting table is long form with number of births in one columns, here with the default name given by stack: "value". Data in this form can be plotted very conveniently with Gadfly.

plot(stack(births, [:Males, :Females]), x=:Year, y=:value, color=:variable,
     Geom.line)
Year 1625 1650 1675 1700 1725 Males Females variable 0 5.0×10³ 1.0×10⁴ value

In some cases, explicitly transforming the data can be burdensome. Gadfly lets you avoid this be referring to columns or groups of columns in a implicit long-form version of the data.

plot(births, x=:Year, y=Col.value(:Males, :Females),
     color=Col.index(:Males, :Females), Geom.line)
Year 1625 1650 1675 1700 1725 Males Females Column 0 5.0×10³ 1.0×10⁴ y

Here Col.value produces the concatenated values from a set of columns, and Col.index refers to a vector labeling each value in that concatenation by the column it came from. Also useful is Row.index, which will give the row index of items in a concatenation.

This syntax also lets us more conveniently plot data that is not in a DataFrame, such as matrices or arrays of arrays. Here we plot each column of a matrix as a separate line.

X = randn(40, 20) * diagm(1:20)
plot(X, x=Row.index, y=Col.value, color=Col.index, Geom.line)
x 0 10 20 30 40 15 5 10 1 20 Column -50 0 50 100 y

When given no arguments Row.index, Col.index, and Col.value assume all columns are being concatenated, but we could have equivalently used Col.index(1:20...), etc.

Plotting arrays of vectors works in much the same way as matrices, but constituent vectors maybe be of varying lengths.

X = [randn(rand(10:20)) for _ in 1:10]
plot(X, x=Row.index, y=Col.value, color=Col.index, Geom.line)
x 0 5 10 15 20 5.0 2.5 7.5 1.0 10.0 Column -3 -2 -1 0 1 2 3 y