Your efforts are commendable, but please note that this task should be less labor intensive. For example we could do:
using DataFrames, Statistics, Plots
# INPUT DATA
df = DataFrame(Age=rand(4), Balance=rand(4), Tenure=rand(4), Salary=rand(4))
cols = [:Age, :Balance, :Tenure] # define subset
M = cor(Matrix(df[!,cols])) # correlation matrix
# PLOT
(n,m) = size(M)
heatmap(M, fc=cgrad([:white,:dodgerblue4]), xticks=(1:m,cols), xrot=90, yticks=(1:m,cols), yflip=true)
annotate!([(j, i, text(round(M[i,j],digits=3), 8,"Computer Modern",:black)) for i in 1:n for j in 1:m])
(NB: added yflip=true for standard orientation of matrix display)
