How do you plot the first few values of a PCA - r

Question

I've run a PCA with a moderately-sized data set, but I only want to visualize a certain amount of points from that analysis because they are from repeat observations and I want to see how close the paired observations are to each other on the plot. I've set it up so that the first 18 individuals are the ones I want to plot, but I can't seem to only plot just the first 18 points without only doing an analysis of only the first 18 instead of the whole data set (43 individuals).
# My data file
TrialsMR<-read.csv("NER_Trials_Matrix_Retrials.csv", row.names = 1)
# I ran the PCA of all of my values (without the categorical variable in col 8)
R.pca <- PCA(TrialsMR[,-8], graph = FALSE)
# When I try to plot only the first 18 individuals with this method, I get an error
fviz_pca_ind(R.pca[1:18,],
labelsize = 4,
pointsize = 1,
col.ind = TrialsMR$Bands,
palette = c("red", "blue", "black", "cyan", "magenta", "yellow", "gray", "green3", "pink" ))
# This is the error
Error in R.pca[1:18, ] : incorrect number of dimensions
The 18 individuals are each paired up, so only using 9 colours shouldn't cause an error (I hope).
Could anyone help me plot just the first 18 points from a PCA of my whole data set?
My data frame looks similar to this in structure
TrialsMR
Trees Bushes Shrubs Bands
JOHN1 1 4 18 BLUE
JOHN2 2 6 25 BLUE
CARL1 1 3 12 GREEN
CARL2 2 4 15 GREEN
GREG1 1 1 15 RED
GREG2 3 11 26 RED
MIKE1 1 7 19 PINK
MIKE2 1 1 25 PINK
where each band corresponds to a specific individual that has been tested twice.

Answer 1

You are using the wrong argument to specify individuals. Use select.ind to choose the individuals required, for eg.:
data(iris) # test data
If you want to rename your rows according to a specific grouping criteria for readily identifiable in a plot. For eg. let setosa lies in series starting with 1, something like in 100-199, similarly versicolor in 200-299 and virginica in 300-399. Do it before the PCA.
new_series <- c(101:150, 201:250, 301:350) # there are 50 of each
rownames(iris) <- new_series
R.pca <- prcomp(iris[,1:4],scale. = T) # pca
library(factoextra)
fviz_pca_ind(X= R.pca, labelsize = 4, pointsize = 1,
select.ind= list(name = new_series[1:120]), # 120 out of 150 selected
col.ind = iris$Species ,
palette = c("blue", "red", "green" ))
Always refer to R documentation first before using a new function.
R documentation: fviz_pca {factoextra}
X
an object of class PCA [FactoMineR]; prcomp and princomp [stats]; dudi and pca [ade4]; expOutput/epPCA [ExPosition].
select.ind, select.var
a selection of individuals/variables to be drawn. Allowed values are NULL or a list containing the arguments name, cos2 or contrib
For your particular dummy data, this should do:
R.pca <- prcomp(TrailsMR[,1:3], scale. = TRUE)
fviz_pca_ind(X= R.pca,
select.ind= list(name = row.names(TrialsMR)[1:4]), # 4 out of 8
pointsize = 1, labelsize = 4,
col.ind = TrialsMR$Bands,
palette = c("blue", "green" )) + ylim(-1,1)
Dummy Data:
TrialsMR <- read.table( text = "Trees Bushes Shrubs Bands
JOHN1 1 4 18 BLUE
JOHN2 2 6 25 BLUE
CARL1 1 3 12 GREEN
CARL2 2 4 15 GREEN
GREG1 1 1 15 RED
GREG2 3 11 26 RED
MIKE1 1 7 19 PINK
MIKE2 1 1 25 PINK", header = TRUE)