带有导入图像的自定义图例

Question

我目前正在通过导入自定义图像并将它们用作 geom_points 在 ggplot2 中创建图，类似于 this post，除了我循环遍历不同的图像以获得因子的独特水平。

有没有简单的方法可以将这些图像添加到图例中？我在 ggplot2 中看到了多篇关于自定义图例的文章，但没有一篇文章涉及导入的图像。

Answer 1

我不确定您将如何生成绘图，但这展示了一种用图像替换图例键的方法。它使用 grid 函数来定位包含图例键 grobs 的视口，并将其中一个替换为 R 徽标

library(png)
library(ggplot2)
library(grid)

# Get image
img <- readPNG(system.file("img", "Rlogo.png", package="png"))

# Plot
p = ggplot(mtcars, aes(mpg, disp, colour = factor(vs))) + 
    geom_point() +
    theme(legend.key.size = unit(1, "cm"))

# Get ggplot grob
gt = ggplotGrob(p)
grid.newpage()
grid.draw(gt)

# Find the viewport containing legend keys
current.vpTree() # not well formatted
formatVPTree(current.vpTree())  # Better formatting - see below for the formatVPTree() function

    # Find the legend key viewports
    # The two viewports are: 
      # key-4-1-1.5-2-5-2
      # key-3-1-1.4-2-4-2

# Or search using regular expressions
Tree = as.character(current.vpTree())
pos = gregexpr("\[key.*?\]", Tree)
match = unlist(regmatches(Tree, pos))

match = gsub("^\[(key.*?)\]$", "\1", match) # remove square brackets
match = match[!grepl("bg", match)]  # removes matches containing bg

# Change one of the legend keys to the image
downViewport(match[2])
grid.rect(gp=gpar(col = NA, fill = "white"))
grid.raster(img, interpolate=FALSE)
upViewport(0)

# Paul Murrell's function to display the vp tree 
formatVPTree <- function(x, indent=0) {
    end <- regexpr("[)]+,?", x)
    sibling <- regexpr(", ", x)
    child <- regexpr("[(]", x)
    if ((end < child || child < 0) && (end < sibling || sibling < 0)) {
        lastchar <- end + attr(end, "match.length")
        cat(paste0(paste(rep("  ", indent), collapse=""), 
                   substr(x, 1, end - 1), "\n"))
        if (lastchar < nchar(x)) {
            formatVPTree(substring(x, lastchar + 1), 
                         indent - attr(end, "match.length") + 1)
        }
    }
    if (child > 0 && (sibling < 0 || child < sibling)) {
        cat(paste0(paste(rep("  ", indent), collapse=""), 
                   substr(x, 1, child - 3), "\n"))
        formatVPTree(substring(x, child + 1), indent + 1)
    }
    if (sibling > 0 && sibling < end && (child < 0 || sibling < child)) {
        cat(paste0(paste(rep("  ", indent), collapse=""), 
                   substr(x, 1, sibling - 1), "\n"))
        formatVPTree(substring(x, sibling + 2), indent)
    }
}

Answer 2

将 html 标签与 element_markdown 结合使用，您可以在图例中指定任何内容。

假设你在一个文件中有一张png图片，这个文件位于某个folder中，文件名image，那么你可以运行，例如：

p = ggplot(mtcars, aes(mpg, disp, colour = factor(vs))) + 
    geom_point() +
    theme(legend.key.size = unit(1, "cm"))
labels <- c()
labels[1]=paste0("<img src='", folder, image, ".png' width='16' />")
labels[2]="<strong>Faces</strong>"
p + theme(legend.text = element_markdown(color = "black", size = 11)) + 
scale_color_manual(name = "factor(vs)", labels = labels, , values = c(1,2))

将return这样：

带有导入图像的自定义图例

Custom legend with imported images

r

legend

ggplot2