r - Text color based on contrast against background

Question

I have the following plot:

m <- structure(list(Var1 = structure(c(1L, 2L, 3L, 4L, 5L, 6L, 7L, 
1L, 2L, 3L, 4L, 5L, 6L, 7L, 1L, 2L, 3L, 4L, 5L, 6L, 7L, 1L, 2L, 
3L, 4L, 5L, 6L, 7L, 1L, 2L, 3L, 4L, 5L, 6L, 7L, 1L, 2L, 3L, 4L, 
5L, 6L, 7L, 1L, 2L, 3L, 4L, 5L, 6L, 7L), .Label = c("FE", "AG", 
"NO", "SPH", "SEP", "H/I", "CMP"), class = "factor"), Var2 = structure(c(1L, 
1L, 1L, 1L, 1L, 1L, 1L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 3L, 3L, 3L, 
3L, 3L, 3L, 3L, 4L, 4L, 4L, 4L, 4L, 4L, 4L, 5L, 5L, 5L, 5L, 5L, 
5L, 5L, 6L, 6L, 6L, 6L, 6L, 6L, 6L, 7L, 7L, 7L, 7L, 7L, 7L, 7L
), .Label = c("FE", "AG", "NO", "SPH", "SEP", "H/I", "CMP"), class = "factor"), 
    value = c(0, 0.0419753086419753, 0.172839506172839, 0.0740740740740741, 
    0.0123456790123457, 0.111111111111111, 0.0617283950617284, 
    0.0419753086419753, 0, 0.0765432098765432, 0.0246913580246914, 
    0.00493827160493827, 0.0567901234567901, 0.0320987654320988, 
    0.172839506172839, 0.0765432098765432, 0, 0.175308641975309, 
    0.0197530864197531, 0.177777777777778, 0.120987654320988, 
    0.0740740740740741, 0.0246913580246914, 0.175308641975309, 
    0, 0.00740740740740741, 0.0814814814814815, 0.0395061728395062, 
    0.0123456790123457, 0.00493827160493827, 0.0197530864197531, 
    0.00740740740740741, 0, 0.0197530864197531, 0.00987654320987654, 
    0.111111111111111, 0.0567901234567901, 0.177777777777778, 
    0.0814814814814815, 0.0197530864197531, 0, 0.0716049382716049, 
    0.0617283950617284, 0.0320987654320988, 0.120987654320988, 
    0.0395061728395062, 0.00987654320987654, 0.0716049382716049, 
    0), vtext = c("0.0%", "4.2%", "17.3%", "7.4%", "1.2%", "11.1%", 
    "6.2%", "4.2%", "0.0%", "7.7%", "2.5%", "0.5%", "5.7%", "3.2%", 
    "17.3%", "7.7%", "0.0%", "17.5%", "2.0%", "17.8%", "12.1%", 
    "7.4%", "2.5%", "17.5%", "0.0%", "0.7%", "8.1%", "4.0%", 
    "1.2%", "0.5%", "2.0%", "0.7%", "0.0%", "2.0%", "1.0%", "11.1%", 
    "5.7%", "17.8%", "8.1%", "2.0%", "0.0%", "7.2%", "6.2%", 
    "3.2%", "12.1%", "4.0%", "1.0%", "7.2%", "0.0%")), .Names = c("Var1", 
"Var2", "value", "vtext"), row.names = c(NA, -49L), class = "data.frame")

library(ggplot2)
ggplot(data = m, aes(x = Var2, y = Var1, fill = value, label = vtext)) +
    xlab("") + ylab("") +
    geom_tile() +
    geom_text() +
    scale_fill_gradient(low="white", high="darkmagenta") +
    # Sample code for subtitles: ggtitle(bquote(atop("Age distribution", atop(italic(.(subtitle)), ""))))
    ggtitle(bquote(atop(.(title), atop(italic(.(subtitle)), "")))) +
    theme(axis.text.y = element_text(size = 12), axis.text.x = element_text(size = 12),
          axis.title = element_text(size = 16, face = "bold"),
          plot.title = element_text(size = 20),
          panel.background = element_rect(fill = "white"),
          legend.key.size = unit(0.02, "npc"),
          legend.text = element_text(size = 14),
          legend.title = element_text(size = 16))

Which results in:

The problem, is that the text in the darker squares is difficult to read. Is it possible to change the text color based on the background color, so the text in the clear boxes is black and in the darker boxes is white?

Answer 1

Add these two code lines:

geom_text(aes(color = value > 0.1)) +
scale_color_manual(guide = FALSE, values = c("black", "white"))

Here text color depends on value (value > 0.1) and colors are specified with scale_color_manual.

For the output like this: