Merge pull request #23 from nanxstats/custom-scale

Add custom color scale function for self-defined color ordering

vignettes/ggsci-faq.Rmd

@@ -13,6 +13,24 @@ vignette: >
   %\VignetteIndexEntry{Frequently Asked Questions about ggsci}
 ---
 
+```{r, include=FALSE}
+knitr::knit_hooks$set(pngquant = knitr::hook_pngquant)
+
+knitr::opts_chunk$set(
+  message = FALSE,
+  collapse = TRUE,
+  comment = "#>",
+  dev = "ragg_png",
+  dpi = 72,
+  fig.retina = 2,
+  fig.width = 3.3334 / 0.618,
+  fig.height = 3.3334,
+  fig.align = "center",
+  out.width = "65%",
+  pngquant = "--speed=1 --quality=50"
+)
+```
+
 ## What if my data has more categories than the number of colors offered?
 
 Although it is recommended that we do not encode too many categories
@@ -54,3 +72,60 @@ options(
 
 p
 ```
+
+## Customize color ordering in a palette
+
+You can customize the color selection and ordering of any discrete
+color palette in ggsci by using the following function that returns
+a custom color scale function. This method is flexible and encourages
+code reuse.
+
+```{r}
+#' Define a custom color scale
+#'
+#' @param pal Name of the color palette, as part of the
+#'   original palette function name.
+#' @param palette Palette type, as defined in the
+#'   original palette function (optional).
+#' @param n Number of (first) colors to fetch from the original palette.
+#' @param order A vector of color index (optional).
+#' @param alpha Transparency level.
+#'
+#' @return A custom color scale function.
+scale_color_custom <- function(pal, palette, n, order, alpha = 1) {
+  pal <- getFromNamespace(paste0("pal_", pal), "ggsci")
+
+  colors <- if (missing(palette)) {
+    pal(alpha = alpha)(n)
+  } else {
+    pal(palette = palette, alpha = alpha)(n)
+  }
+
+  if (length(order) > length(colors)) {
+    stop("The length of order exceeds the number of colors.", call. = FALSE)
+  }
+  colors <- if (!missing(order)) colors[order]
+
+  ggplot2::scale_color_manual(values = colors)
+}
+```
+
+Use `scale_color_custom()` in an example:
+
+```{r}
+library(ggplot2)
+library(ggsci)
+
+set.seed(42)
+df <- data.frame(
+  x = rnorm(100),
+  y = rnorm(100),
+  group = factor(sample(1:5, 100, replace = TRUE))
+)
+
+p <- ggplot(df, aes(x = x, y = y, color = group)) +
+  geom_point(size = 3) +
+  theme_minimal()
+
+p + scale_color_custom("d3", palette = "category20", n = 20, order = c(14, 11, 13, 12, 15))
+```