heatmap using scatterplot

df1 <- expand.grid(1:5,1:5)

df1$Var3 <- (1:25)^2

head(df1, 4)

plot(df1[,1:2], pch=15, col=ifelse(df1$Var3>max(df1$Var3)*1/3, ifelse(df1$Var3>max(df1$Var3)*2/3,"red", "green") , "blue"), cex=10, main="heatmap of frequency")

text(df1[,1:2],labels=df1$Var3, col="white")