Create Heatmap for Lower Triangular Matrix in R

A heatmap is a diagrammatic representation of data where the values are represented with colours. Mostly, it is used to display data that has slight variation. We can draw it for a full matrix, an upper triangular matrix as well as a lower triangular matrix. This can be done with the help of image function.

Example1

Live Demo

> M1<-matrix(rpois(36,5),ncol=6)
> M1

Output

   [,1] [,2] [,3] [,4] [,5] [,6]
[1,] 6    9    4    7    5    4
[2,] 6    6    4    3    7    5
[3,] 2    6    1    2    2    5
[4,] 5    6    2    4    3    5
[5,] 2   10    3    4    5    6
[6,] 5    6    6    4    4    3

Example

> M1[upper.tri(M1)]=NA
> M1

Output

   [,1] [,2] [,3] [,4] [,5] [,6]
[1,] 6   NA   NA   NA   NA   NA
[2,] 6    6   NA   NA   NA   NA
[3,] 2    6   1    NA   NA   NA
[4,] 5    6   2    4    NA   NA
[5,] 2   10   3    4     5   NA
[6,] 5    6   6    4     4    3

Example

> image(1:6,1:6,M1)

Output:

Example2

Live Demo

> M2<-matrix(rnorm(25,1,0.24),nrow=5)
> M2

Output

     [,1]         [,2]       [,3]     [,4]     [,5]
[1,] 1.0839897 0.4979137 0.8792797 1.0867854 1.067157
[2,] 0.9422221 1.2385106 0.8464642 1.0249176 0.689996
[3,] 1.0664669 0.7912711 1.1352022 1.2694327 1.200055
[4,] 1.2765053 1.2013291 1.0647500 0.9837964 1.181706
[5,] 1.1613147 1.2735703 1.4184856 1.1998669 1.072919

Example

> M2[upper.tri(M2)]=NA
> M2

Output

[,1] [,2] [,3] [,4] [,5]
[1,] 1.0839897 NA NA NA NA
[2,] 0.9422221 1.2385106 NA NA NA
[3,] 1.0664669 0.7912711 1.135202 NA NA
[4,] 1.2765053 1.2013291 1.064750 0.9837964 NA
[5,] 1.1613147 1.2735703 1.418486 1.1998669 1.072919

Example

> image(1:5,1:5,M2)

Output: