画一条垂直线与一条水平线交汇

Question

I would like to draw a vertical line at a certain/arbitrary point to meet a certain horizontal line:

xyplot(Amplification ~ Voltage | Serial_number,
       data = APD[APD$Serial_number==912009897,],
       panel = function(x, ...){
                                panel.xyplot(x, ...);
                                panel.abline(h = 150)
                                panel.abline(v = 350)},
       ylim = c(100,200),
       grid = TRUE 
)

Perfectly both lines will end at the crossing to make it more descriptive. How can I do that? Thank you!

Answer 1

So, if you want x = 350 and y = 150, as the end of the lines I would go through command 'points' and make two lines, just guessing some safe starting points for both lines:

points(c(350,350),c(100, 150), type='l') # the vertical one

points(c(100, 350),c(150, 150), type='l') # the horizontal one

In a more generalised way, you first define your point and after plotting, you get the axis dimensions and make the lines with the actual values:

point <- c(350,150) # point of line crossing
plot(.... ) # your plot
mrs <- par('usr') # axis limits
## Now the lines
points(rep(point[1],2),c(mrs[3], point[2]), type='l')
points(c(mrs[1], point[1]),rep(point[2],2), type='l')

that should do the job.

the lines at the end will make a line between two points, it is something like joining (x1,y1) to (x2,y2). So, with the command par('usr') you will get the starting and ending points of both axis, in a vector of length 4, something like (min(x),max(x),min(y),max(y)). so with points you just make a line bettwen (c(x1,x2),c(y1,y2)). For the vertical one, then your x-coordinate is the same for both points, that's why there is rep(point[1],2), and in the y, it is the value of y and the starting of y axis (that is why it is msr[3], is the third number of the vector), and so on...