High Dimensional Data Visualization

Serialaxes coordinate

Serial axes coordinate is a methodology for visualizing the \(p\)-dimensional geometry and multivariate data. As the name suggested, all axes are shown in serial. The axes can be a finite \(p\) space or transformed to an infinite space (e.g. Fourier transformation).

In the finite \(p\) space, all axes can be displayed in parallel which is known as the parallel coordinate; also, all axes can be displayed under a polar coordinate that is often known as the radial coordinate or radar plot. In the infinite space, a mathematical transformation is often applied. More details will be explained in the sub-section Infinite axes

A point in Euclidean \(p\)-space \(R^p\) is represented as a polyline in serial axes coordinate, it is found that a point <–> line duality is induced in the Euclidean plane \(R^2\) (A. Inselberg and Dimsdale 1990).

Before we start, a couple of things should be noticed:

In the serial axes coordinate system, no x or y (even group) are required; but other aesthetics, such as colour, fill, size, etc, are accommodated.
Layer geom_path is used to draw the serial lines; layer geom_histogram, geom_quantiles, and geom_density are used to draw the histograms, quantiles (not quantile regression) and densities. Users can also customize their own layer (i.e. geom_boxplot, geom_violin, etc) by editing function add_serialaxes_layers.

Finite axes

Suppose we are interested in the data set iris. A parallel coordinate chart can be created as followings:

library(ggmulti)
# parallel axes plot
ggplot(iris, 
       mapping = aes(
         Sepal.Length = Sepal.Length,
         Sepal.Width = Sepal.Width,
         Petal.Length = Petal.Length,
         Petal.Width = Petal.Width,
         colour = factor(Species))) +
  geom_path(alpha = 0.2)  + 
  coord_serialaxes() -> p
p

A histogram layer can be displayed by adding layer geom_histogram

p + 
  geom_histogram(alpha = 0.3, 
                 mapping = aes(fill = factor(Species))) + 
  theme(axis.text.x = element_text(angle = 30, hjust = 0.7))

A density layer can be drawn by adding layer geom_density

p + 
  geom_density(alpha = 0.3, 
               mapping = aes(fill = factor(Species)))

A parallel coordinate can be converted to radial coordinate by setting axes.layout = "radial" in function coord_serialaxes.

p$coordinates$axes.layout <- "radial"
p

Note that: layers, such as geom_histogram, geom_density, etc, are not implemented in the radial coordinate yet.

Infinite axes

Andrews (1972) plot is a way to project multi-response observations into a function \(f(t)\), by defining \(f(t)\) as an inner product of the observed values of responses and orthonormal functions in \(t\)

\[f_{y_i}(t) = <\mathbf{y}_i, \mathbf{a}_t>\]

where \(\mathbf{y}_i\) is the \(i\)th responses and \(\mathbf{a}_t\) is the orthonormal functions under certain interval. Andrew suggests to use the Fourier transformation

\[\mathbf{a}_t = \{\frac{1}{\sqrt{2}}, \sin(t), \cos(t), \sin(2t), \cos(2t), ...\}\]

which are orthonormal on interval \((-\pi, \pi)\). In other word, we can project a \(p\) dimensional space to an infinite \((-\pi, \pi)\) space. The following figure illustrates how to construct an “Andrew’s plot”.

p <- ggplot(iris, 
            mapping = aes(Sepal.Length = Sepal.Length,
                          Sepal.Width = Sepal.Width,
                          Petal.Length = Petal.Length,
                          Petal.Width = Petal.Width,
                          colour = Species)) +
  geom_path(alpha = 0.2, 
            stat = "dotProduct")  + 
  coord_serialaxes()
p

High Dimensional Data Visualization

Wayne Oldford and Zehao Xu

2024-04-08

Serialaxes coordinate

Finite axes

Infinite axes