Question

I have an ggplot2 point chart created like this

qplot(Index, Popularity ,data = data.slopeone.agg)

now I would like to add a vertical line at the half of all data points. In other words, where the integral is 0.5. I want add this line to see wich part of the data makes 50%.

How can I achieve this in R? I know about geom_vline but don’t know how to determine the positon of the vline.

The data is structured like this:
Each row has an ItemId the popularity of this item and an index for my chart to display the values sorted by popularity.

head(data.slopeone.agg)
    Item Popularity Index
184  258 0.07695880     1
29    50 0.07294129     2
121  181 0.07162558     3
203  286 0.07030986     4
225  313 0.06500478     5
65   100 0.06366796     6

my chart looks like this: http://img838.imageshack.us/img838/3194/popt.png

Answer 1

I now found out how to solve my problem. Maybe it’s not the most elegant way:

define a function to get the 50% of popularity data vertical line position

getPopularityVLineIndex = function(popData){

  halfPopularity = sum(popData$Popularity) / 2

  # initialize helper variables
  fiftyPercent = 0;
  counter = 1;

  # sum up the popularity values until the half of the sum is reached
  while (fiftyPercent < halfPopularity) {
      fiftyPercent = fiftyPercent + popData$Popularity[which(popData$Index == counter)]
      counter = counter + 1
  }

  # the current counter value is the position of the vertical line
  vLineIndex = counter

  return (vLineIndex)
}

qplot(Index, Popularity ,data = popData) + geom_vline(xintercept = getPopularityVLineIndex(popData), colour="red", linetype = "longdash"))

If somebody knows a more elegant way, feel free to post it. But maybe my question is now more understandable 😉