David@Work blog shows how to fill in the area between two crossing lines in an Excel chart. This post was also published as a guest-post on PTS blog.

Let’s try to replicate this graph in ggplot2.

First, load ggplot2 and generate the data frame to be used in the example (I am using a slightly modified dataset, therefore the final result will differ somewhat from the original graph).

> library(ggplot2)

> cross <- data.frame(x1 = c(2, 3.27, 6.26, 7.58,
     8.33, 9.79, 11.2, 13.86), y1 = c(13, 14,
     15, 42, 10, 41, 23, 20), y2 = c(37, 18,
     19, 28, 14, 21, 29, 25))

Filling just the area between the two lines is accomplished easily in ggplot2, however as we would need the segments to be of different colour then some extra work is required.

> ggplot(cross, aes(x1, ymin = y1, ymax = y2)) +
     geom_ribbon()

In order to change the fill colour at each point where two lines cross, the points of intersection need to be calculated.

> cross$slope1 <- c(NA, with(cross, diff(y1)/diff(x1)))
> cross$slope2 <- c(NA, with(cross, diff(y2)/diff(x1)))
> cross$intcpt1 <- with(cross, y1 - slope1 *
     x1)
> cross$intcpt2 <- with(cross, y2 - slope2 *
     x1)
> cross$x2 <- with(cross, (intcpt1 - intcpt2)/(slope2 -
     slope1))
> cross$y3 <- with(cross, slope1 * x2 + intcpt1)
> cross <- cross[, c(-4:-7)]

Now, just as an extra precaution and to make sure that calculations are correct, we check visually the location of the points of intersection:

> ggplot(cross) + geom_line(aes(x1, y1), colour = "red") +
     geom_line(aes(x1, y2), colour = "darkblue") +
     geom_point(aes(x2, y3), colour = "lightgreen",
         size = 4)

As I am planning to colour the plot above generated using geom_ribbon the points of intersection need also to be presented in the form expected by geom_ribbon (x, ymin, ymax) – a simple copy of y3 accomplishes this.

> cross$y4 <- cross$y3

Additional error-checking is also obviously needed, as is indicated by the position of the left and rightmost green dots on the above graph – any two lines can have a point of intersection which falls outside the limits of the particular plot.

> cross[which(cross$x2 > cross$x1), c("x2", "y3",
     "y4")] <- NA
> cross$segment <- findInterval(cross$x1, c(cross$x2[which(!is.na(cross$x2))]))

For ggplot2 to be able to vary the fill colour at each crossing of the lines it needs to know the start and end point of each coloured area. This means that the middle points of intersection need to be duplicated, as they would be part of two adjacent areas filled with different colours.

> cross$x3 <- c(tail(cross$x2, -1), NA)
> cross$y5 <- c(tail(cross$y3, -1), NA)
> cross$y6 <- cross$y5

Now the coordinates of two lines and the start/end points of coloured areas need to be combined into one dataframe in a long format.

> cross1 <- cross[, c(1:3, 7)]
> cross2 <- cross[!is.na(cross$x2), c(4:6, 7)]
> cross3 <- cross[!is.na(cross$x3), c(8:10, 7)]

> names(cross2) <- names(cross1)
> names(cross3) <- names(cross1)

> combo <- rbind(cross1, cross2)
> combo <- rbind(combo, cross3)
> combo <- combo[is.finite(combo$y1), ]
> combo <- combo[order(combo$x1), ]

> ggplot(combo, aes(x1, ymin = y1, ymax = y2)) +
     geom_ribbon(aes(fill = factor(segment)))

Each segment is filled with a different colour, but we want to limit the number of fill colours to two.

> ggplot(combo, aes(x1, ymin = y1, ymax = y2,
     )) + geom_ribbon(aes(fill = factor(segment%%2))) +
     geom_path(aes(y = y1), colour = "red",
         size = 1) + geom_path(aes(y = y2),
     colour = "darkblue", size = 1) + opts(legend.position = "none")