在晶格xyplot中绘制每组面板数据的第一点 [英] plot the first point of each group of panel data in lattice xyplot

问题描述

我正在尝试使用组和面板创建折线图，在每个组的第一个值上叠加一个符号.此尝试仅绘制第一个组的第一个点，而对其他两个组不做任何事情.

I'm trying to create a line plot using groups and panels that superposes a symbol on the first value of each group. This attempt plots the first point of the first group only and does nothing for the other two groups.

library(lattice)
foo <- data.frame(x=-100:100, y=sin(c(-100:100)*pi/4))
xyplot( y~x | y>0,  foo, groups=cut(x,3),
        panel = function(x, y, subscripts, ...) {
          panel.xyplot(x, y, subscripts=subscripts, type='l', ...)
          # almost but not quite:
          panel.superpose(x[1], y[1], subscripts=subscripts, cex=c(1,0), ...) 
        } )

可以理解一般解决方案，以允许在每个组和面板中绘制特定点(例如，第一，中间和端点).

General solutions would be appreciated, to allow plotting of specific points within each group and panel (e.g., first, middle, and endpoints).

推荐答案

(感谢这个很好的lattice问题.)您应该使用下标，因为它是为面板选择单个数据点的机制:在这里，您想选择groups by面板:groups[subscripts].一旦有了正确的分组变量，就可以使用它来拆分数据并选择每个组的第一个元素:

(Thanks for this good lattice question.) You should use Subscripts because it is the mechanism for picking individual data points for panels : Here you want to pick the groups by panel: groups[subscripts]. Once you have the right grouping variables you can use it to split your data and pick the first element of each group:

    ## first points
    xx = tapply(x,groups[subscripts],'[',1) 
    yy = tapply(y,groups[subscripts],'[',1)
    ## end points
    xx = tapply(x,groups[subscripts],tail,1) 
    yy = tapply(y,groups[subscripts],tail,1)

您使用panel.points(比基本panel.superpose更高的水平)绘制点.

The you plot the point using panel.points (higher level than the basic panel.superpose).

library(lattice)
foo <- data.frame(x=-100:100, y=sin(c(-100:100)*pi/4))
xyplot( y~x | y>0,  foo, groups=cut(x,3),
        panel = function(x, y, subscripts, groups,...) {
          panel.xyplot(x, y, subscripts=subscripts, type='l',groups=groups, ...)
          # Note the use `panel.points` and the grouping using groups[subscripts]
          panel.points(tapply(x,groups[subscripts],'[',1), 
                       tapply(y,groups[subscripts],'[',1), 
                       cex=2,pch=20, ...) 
        } )

如果要根据颜色组对点进行着色，则应在panel.points中添加一个groups参数. (我把这个留给你练习)

In case you want to color points according the color groups, you should add a groups argument to panel.points. (I leave you this as an exercise)

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