17、数据可视化：多种图形的创建与应用

数据可视化：多种图形的创建与应用

1. 创建向量场

1.1 问题描述

向量场数据存在两个问题：一是数据分辨率过高难以读取；二是线段没有箭头指示流向。

1.2 解决方案

• 降低数据分辨率：定义 every_n() 函数，每隔 n 个值保留一个，其余丢弃。

# Take a slice where z is equal to the minimum value of z
islice <- subset(isabel, z == min(z))
# Keep 1 out of every 'by' values in vector x
every_n <- function(x, by = 2) {
    x <- sort(x)
    x[seq(1, length(x), by = by)]
}
# Keep 1 of every 4 values in x and y
keepx <- every_n(unique(isabel$x), by=4)
keepy <- every_n(unique(isabel$y), by=4)
# Keep only those rows where x value is in keepx and y value is in keepy
islicesub <- subset(islice, x %in% keepx  &  y %in% keepy)

• 绘制带箭