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*) and densities. Users can also customize their own layer (i.e.`quantile`

regression`geom_boxplot`

,`geom_violin`

, etc) by editing function`add_serialaxes_layers`

.

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`

A parallel coordinate can be converted to radial coordinate by
setting `axes.layout = "radial"`

in function
`coord_serialaxes`

.

Note that: layers, such as
`geom_histogram`

, `geom_density`

, etc, are not
implemented in the radial coordinate yet.

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”.