Patrick Patrick - 3 months ago 19
R Question

2-D Scatter Plot with Gradient Color Legend on Top

I am trying to display a 2-D colored scatter plot -- i.e, Y as function of X with point colors driven by Z.

I would like the Z color-gradient legend to be on top, ideally below the main plot title.

I inspired myself from this SO with Z color-gradient on the right to derive the following function:

library(grDevices)
library(colorRamps)

# 2-dim scatter plot with color gradient legend on top.
scatterPlot2DWithColorLegend <- function(x, y, z, colorGradient, legendTitle=""
, main="", xlab="", ylab="", pch=16, cex=1)
{
parPrev <- par()
nColorGradient <- length(colorGradient)
labelRange <- zRange <- range(z)
layout(matrix(1:2, nrow=2), widths = c(1), heights = c(1, 2), FALSE) # 2 plots, one above the other.
# Plot legend first on top.
plot(c(0, 1), c(0, 0.05), type = 'n', axes = F, xlab = '', ylab = '', main = legendTitle, cex.main=0.5)
legend_image <- as.raster(matrix(colorGradient, nrow = 1))
text(x = seq(0, 1, l = 5), y = 0.1 , labels = seq(labelRange[1], labelRange[2], l = 5), pos=1)
rasterImage(legend_image, 0, 0, 1, 1)
# Main plot second on bottom.
if (1 < length(unique(zRange)))
colVec = colorGradient[as.numeric(cut(z, nColorGradient))]
else
colVec = colorGradient[1]
plot(x, y, col = colVec
, main = main, xlab = xlab, ylab = ylab, pch = pch, cex = cex)
par(parPrev)
}


This is a simple test code:

# Test data.
mdf <- data.frame(X=c(0:10))
mdf$Y <- mdf$X * 3
mdf$Z <- (mdf$X-5)^2

# Color gradient function.
colorGradient <- colorRampPalette(c("blue", "green", "yellow", "red"))(4)

# 2-D colored scatter plot.
scatterPlot2DWithColorLegend(mdf$X, mdf$Y, mdf$Z, colorGradient
, legendTitle="Z", main="Y vs. X with Z-Color", ylab="Y", xlab="X"
, pch=16, cex=0.7)


With my
scatterPlot2DWithColorLegend
function as above, I get:

enter image description here

I desire something like:

enter image description here

Can someone quickly offer an enhanced version of the scatterPlot2DWithColorLegend function or point me to an existing package/function that would get me what I want? Namely:


  1. Main title above the color-gradient legend (Y vs. X with Z-Color).

  2. Z-color gradient image smaller, height and width.

  3. Z-color gradient Z-range on bottom (the Z-range does not currently show).

  4. Z-color gradient legend to the left of Z-color gradient image as opposed to top (i.e., legendTitle="Z").

  5. Significantly reduce spacing between main title, color-gradient legend, and the main scatter plot.



Obviously, I don't understand well R graphics. I am not familiar with the lattice, ggplot and the kinds either - the number of options appears overwhelming. I would like something simple that works I can reuse to get over this specific hump as this seems very basic.

Thanks in advance for any help.

Answer

Here's a ggplot alternative:

ggplot(data = mdf, aes(x = X, y = Y, col = Z)) +
  geom_point() +
  scale_colour_gradientn(colours = colourGradient) +
  theme_bw() +
  theme(legend.position = "top") +
  ggtitle("Y vs. X with Z-Color")

enter image description here