ggplot2可以找到交叉点 - 还是有其他整齐的方式？

Question

在实验中，在几个时间点测量血压。实验期间血压上升和下降。我需要绘制血压反应（简单部分）并找出血压加倍的时间点（x值）（棘手的部分）。我想知道是否可以在ggplot中检索这些信息？

以下是一个例子：

# Generate data
time <- c(10, 60, 90, 200, 260, 300, 700)
value <- c(1, 6, 8, 40, 50, 60, 70)
df <- data.frame(time, value)

# The first value of "value" is the first observation.
# When the first "value" increased ten times, it is equal to 10
# Question is at what time point did the value increase ten times according to the graph?

ggplot(data=c, aes(x=time, y=value,)) + 
     geom_line() +
     geom_hline(y=10, colour="red") +
     annotate("text", hjust=0, x=170, y=15, label="I need to find the x value at the intersection")

任何解决方案？

Answer 1

不，这不能用ggplot2完成。但是，这很容易做到：

v0 <- 10

f1 <- approxfun(df$time, df$value)

#we use numeric optimization here, but an analytical solution is of course possible (though a bit more work)
#this finds only one intersection, more work is required if there are more than one
optimize(function(t0) abs(f1(t0) - v0), interval = range(df$time))
#$minimum
#[1] 96.87501
#
#$objective
#[1] 3.080161e-06

如果你的数据是双射的，那就更简单了：

f2 <- approxfun(df$value, df$time)
f2(v0)
#[1] 96.875