Ggplot: How to create a gradient fill in a geomagpoly

Question

It should be pretty easy, but I can't find my way through.

tri_fill <- structure( list(x= c(0.75, 0.75, 2.25, 3.25), y = c(40, 43, 43, 40)), .Names = c("x", "y"), row.names = c(NA, -4L), class = "data.frame",Integrated=NA, Related=NA) # install.packages("ggplot2", dependencies = TRUE) require(ggplot2) ggplot(data=tri_fill,aes(x=x, y=y))+ geom_polygon() + scale_fill_gradient(limits=c(1, 4), low = "lightgrey", high = "red") 

What I want is a gradient along the x axis, but with the above I get only a legend with a gradient and a polygon with a solid fill.

Answer 1

Here is a possible solution when you have a relatively simple training ground. Instead of a polygon, we create many line segments and color them with a gradient. Thus, the result will look like a polygon with a gradient.

 #create data for 'n'segments n_segs <- 1000 #x and xend are sequences spanning the entire range of 'x' present in the data newpolydata <- data.frame(xstart=seq(min(tri_fill$x),max(tri_fill$x),length.out=n_segs)) newpolydata$xend <- newpolydata$xstart #y are a little more complicated: when x is below changepoint, y equals max(y) #but when x is above the changepoint, the border of the polygon #follow a line according to the formula y= intercept + x*slope. #identify changepoint (very data/shape dependent) change_point <- max(tri_fill$x[which(tri_fill$y==max(tri_fill$y))]) #calculate slope and intercept slope <- (max(tri_fill$y)-min(tri_fill$y))/ (change_point - max(tri_fill$x)) intercept <- max(tri_fill$y) #all lines start at same y newpolydata$ystart <- min(tri_fill$y) #calculate y-end newpolydata$yend <- with(newpolydata, ifelse (xstart <= change_point, max(tri_fill$y),intercept+ (xstart-change_point)*slope)) p2 <- ggplot(newpolydata) + geom_segment(aes(x=xstart,xend=xend,y=ystart,yend=yend,color=xstart)) + scale_color_gradient(limits=c(0.75, 4), low = "lightgrey", high = "red") p2 #note that I've changed the lower border of the gradient. 

EDIT: the above solution works if only a polygon with a gradient is required, however, as mentioned in the comments, this can lead to problems when you planned to match one thing to fill and another thing to color, aes' can be used only once. Therefore, I modified the solution so as not to build lines, but to build (very thin) polygons that may have filling.

 #for each 'id'/polygon, four x-variables and four y-variable #for each polygon, we start at lower left corner, and go to upper left, upper right and then to lower right. n_polys <- 1000 #identify changepoint (very data/shape dependent) change_point <- max(tri_fill$x[which(tri_fill$y==max(tri_fill$y))]) #calculate slope and intercept slope <- (max(tri_fill$y)-min(tri_fill$y))/ (change_point - max(tri_fill$x)) intercept <- max(tri_fill$y) #calculate sequence of borders: x, and accompanying lower and upper y coordinates x_seq <- seq(min(tri_fill$x),max(tri_fill$x),length.out=n_polys+1) y_max_seq <- ifelse(x_seq<=change_point, max(tri_fill$y), intercept + (x_seq - change_point)*slope) y_min_seq <- rep(min(tri_fill$y), n_polys+1) #create polygons/rectangles poly_list <- lapply(1:n_polys, function(p){ res <- data.frame(x=rep(c(x_seq[p],x_seq[p+1]),each=2), y = c(y_min_seq[p], y_max_seq[p:(p+1)], y_min_seq[p+1])) res$fill_id <- x_seq[p] res } ) poly_data <- do.call(rbind, poly_list) #plot, allowing for both fill and color-aes p3 <- ggplot(tri_fill, aes(x=x,y=y))+ geom_polygon(data=poly_data, aes(x=x,y=y, group=fill_id,fill=fill_id)) + scale_fill_gradient(limits=c(0.75, 4), low = "lightgrey", high = "red") + geom_point(aes(color=factor(y)),size=5) p3