November 2019
Intermediate to advanced
472 pages
9h 23m
Japanese
Content preview from Rグラフィックスクックブック 第2版 ―ggplot2によるグラフ作成のレシピ集
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198
8
章 軸
レシピ
8.4
カテゴリカルな軸の要素の順番を変更する
問題
カテゴリカルな軸の要素の順番を変更したい。
解決策
カテゴリカルな軸（離散値軸）では、ファクタの
1
要素が軸にマッピングされています。要素の順番
は
scale_x_discrete()
や
scale_y_discrete()
の
limits
を設定することで変更することができます。
手動で軸の要素の順番を設定するには、変更したい順番にレベルを並べたベクトルを
limits
に指定
します。このベクトルには、いくつかの要素を除外して指定することもできます（図8-8）。
pg_plot <- ggplot(PlantGrowth, aes(x = group, y = weight)) +
geom_boxplot()
pg_plot +
scale_x_discrete(limits = c("trt1", "ctrl", "trt2"))
解説
この方法は軸の要素の一部だけを図に表示するためにも使えます。次のようにすると
ctrl
と
trt1
だ
けが表示されます（図 8-8右）。この際、データが削除されているため、警告が出ます。
pg_plot +
scale_x_discrete(limits = c("ctrl", "trt1"))
#> Warning: Removed 10 rows containing missing values (stat_boxplot).
図8-8 左：手動でx軸の要素を指定した箱ひげ図 右：2つの要素だけに
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