PCA Biplot with `ggplot2`

Produces a ggplot2 variant of a so-called biplot for PCA (principal component analysis), but is more flexible and more appealing than the base R biplot() function.

Usage

ggplot_pca(x, choices = 1:2, scale = 1, pc.biplot = TRUE,
  labels = NULL, labels_textsize = 3, labels_text_placement = 1.5,
  groups = NULL, ellipse = TRUE, ellipse_prob = 0.68,
  ellipse_size = 0.5, ellipse_alpha = 0.5, points_size = 2,
  points_alpha = 0.25, arrows = TRUE, arrows_colour = "darkblue",
  arrows_size = 0.5, arrows_textsize = 3, arrows_textangled = TRUE,
  arrows_alpha = 0.75, base_textsize = 10, ...)

Source

The ggplot_pca() function is based on the ggbiplot() function from the ggbiplot package by Vince Vu, as found on GitHub: https://github.com/vqv/ggbiplot (retrieved: 2 March 2020, their latest commit: 7325e88; 12 February 2015).

As per their GPL-2 licence that demands documentation of code changes, the changes made based on the source code were:

Rewritten code to remove the dependency on packages plyr, scales and grid
Parametrised more options, like arrow and ellipse settings
Hardened all input possibilities by defining the exact type of user input for every argument
Added total amount of explained variance as a caption in the plot
Cleaned all syntax based on the lintr package, fixed grammatical errors and added integrity checks
Updated documentation

Arguments

x:

An object returned by pca(), prcomp() or princomp().
choices:

length 2 vector specifying the components to plot. Only the default is a biplot in the strict sense.
scale:

The variables are scaled by lambda ^ scale and the observations are scaled by lambda ^ (1-scale) where lambda are the singular values as computed by princomp. Normally 0 <= scale <= 1, and a warning will be issued if the specified scale is outside this range.
pc.biplot:

If true, use what Gabriel (1971) refers to as a "principal component biplot", with lambda = 1 and observations scaled up by sqrt(n) and variables scaled down by sqrt(n). Then inner products between variables approximate covariances and distances between observations approximate Mahalanobis distance.
labels:

An optional vector of labels for the observations. If set, the labels will be placed below their respective points. When using the pca() function as input for x, this will be determined automatically based on the attribute non_numeric_cols, see pca().
labels_textsize:

The size of the text used for the labels.
labels_text_placement:

Adjustment factor the placement of the variable names (>=1 means further away from the arrow head).
groups:

An optional vector of groups for the labels, with the same length as labels. If set, the points and labels will be coloured according to these groups. When using the pca() function as input for x, this will be determined automatically based on the attribute non_numeric_cols, see pca().
ellipse:

A logical to indicate whether a normal data ellipse should be drawn for each group (set with groups).
ellipse_prob:

Statistical size of the ellipse in normal probability.
ellipse_size:

The size of the ellipse line.
ellipse_alpha:

The alpha (transparency) of the ellipse line.
points_size:

The size of the points.
points_alpha:

The alpha (transparency) of the points.
arrows:

A logical to indicate whether arrows should be drawn.
arrows_colour:

The colour of the arrow and their text.
arrows_size:

The size (thickness) of the arrow lines.
arrows_textsize:

The size of the text at the end of the arrows.
arrows_textangled:

A logical whether the text at the end of the arrows should be angled.
arrows_alpha:

The alpha (transparency) of the arrows and their text.
base_textsize:

The text size for all plot elements except the labels and arrows.
...:

Arguments passed on to functions.

Details

The colours for labels and points can be changed by adding another scale layer for colour, such as scale_colour_viridis_d() and scale_colour_brewer().

Examples

# `example_isolates` is a data set available in the AMR package.
# See ?example_isolates.

# \donttest{
if (require("dplyr")) {
  # calculate the resistance per group first
  resistance_data <- example_isolates %>%
    group_by(
      order = mo_order(mo), # group on anything, like order
      genus = mo_genus(mo)
    ) %>% #   and genus as we do here;
    filter(n() >= 30) %>% # filter on only 30 results per group
    summarise_if(is.sir, resistance) # then get resistance of all drugs

  # now conduct PCA for certain antimicrobial drugs
  pca_result <- resistance_data %>%
    pca(AMC, CXM, CTX, CAZ, GEN, TOB, TMP, SXT)

  summary(pca_result)

  # old base R plotting method:
  biplot(pca_result, main = "Base R biplot")

  # new ggplot2 plotting method using this package:
  if (require("ggplot2")) {
    ggplot_pca(pca_result) +
      labs(title = "ggplot2 biplot")
  }
  if (require("ggplot2")) {
    # still extendible with any ggplot2 function
    ggplot_pca(pca_result) +
      scale_colour_viridis_d() +
      labs(title = "ggplot2 biplot")
  }
}
#> Warning: There were 73 warnings in `summarise()`.
#> The first warning was:
#> ℹ In argument: `PEN = (function (..., minimum = 30, as_percent = FALSE,
#>   only_all_tested = FALSE) ...`.
#> ℹ In group 5: `order = "Lactobacillales"` `genus = "Enterococcus"`.
#> Caused by warning:
#> ! Introducing NA: only 14 results available for PEN in group: order =
#> "Lactobacillales", genus = "Enterococcus" (`minimum` = 30).
#> ℹ Run `dplyr::last_dplyr_warnings()` to see the 72 remaining warnings.
#> ℹ Columns selected for PCA: "AMC", "CAZ", "CTX", "CXM", "GEN", "SXT",
#>   "TMP", and "TOB". Total observations available: 7.
#> Groups (n=4, named as 'order'):
#> [1] "Caryophanales"    "Enterobacterales" "Lactobacillales"  "Pseudomonadales" 
#> 


# }

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PCA Biplot with ggplot2