Finding "first" values for line chart with cumulative percentage

Hi, I have a set of data like below.  These are tasks that happen during a specific week and take a number of days to complete.  I'm trying to build a line chart that would have 3 lines and show by week how many days it took for the tasks to be 50% complete, 80% complete and finally 100% complete.  I can get the accumulated percentages for each day value by week in a pivot table but I can't come up with a way to take the first number of days value for a particular percentage and chart it.  For example in the data below, I know for the week of 7/30 that after 1 day the tasks were 33.33% complete, after 2 days 66.66% complete, after 4 days 83.33% complete and after 5 days 100% complete.  I'd like to chart for that week that it took 2 days to get to (or above) 50% complete, 4 days to 80%, and 5 days to 100%.  The three percentage buckets would  be separate lines with the y values as the number of days, and the x values as the week starting.

Thanks in advance for any ideas.