R programming tutorial: How to create heatmap with ggplot2 in R

A heatmap is just a grip type data visualization of the correlation matrix between each variable pair of a data frame. In R, if you want to create a heatmap, first create a correlation matrix, then transform the matrix to a long form data set. Using the geom_tile() from ggplot2 package, a heatmap can be plotted.
#rprogramming
#rstudio
#ggplot2
#heapmap
#correlationmatrix
#rdatacode