I just add geom_subtile() and geom_subrect() function in package ggalign to subdivide rectangles with shared borders into a grid.

library(reshape2)
library(ggplot2)
m <- matrix(
    c("SNV", "SNV", NA, NA, "INDEL", "SNV", "INDEL", "SNV", "SNV/INDEL"),
    3, 3
)

ggplot(
    tidyr::separate_longer_delim(melt(m), value, delim = "/"),
    aes(Var1, Var2, fill = value)
) +
    geom_subtile(color = "yellow", linewidth = 2) +
    xlab("Patient") +
    ylab("Gene")

If you want to draw the oncoplot, ggalign provide more advanced function to do this, it can do everything ComplexHeatmap can, and even more!

The oncoplot vignette is here: https://yunuuuu.github.io/ggalign/dev/articles/oncoplot.html