Update: With Stengel in the office, I updated the graph and the data. I also added three dates for Richard Stengel’s tenure in the office based on the shortest, average, and longest tenures Undersecretaries before him had. Here, I am not trying to forecast when he is going to leave his post. Rather, I plan to use these dates to make arguments about vacancies and high rate of turnover in R.
Let me share a short post about the vacancy data about the Undersecretary for Public Affairs and Public Diplomacy position. Everybody who works in or studies public diplomacy knows that the position has seen high rates of turn over and has been vacant quite often. I was playing around with the vacancy data and created this timeline visual.
Here is the data I used as a table.
Reaching the Public 
Evin — Very useful. Thank you. John
It is missing the period where Ann Stock was acting Undersec after McHale’s Navy. She filled in a chunk of time before Kathleen Stephens.
Paul, do you have the exact dates? I would love to update the timeline.
Actually, I’d leave the chart alone. I don’t think Ann Stock was “acting” under secretary. Generally speaking, State does not name acting under secretaries. Ms. Stock was designated to hold certain of the under secretary’s authorities — mostly for administrative purposes — but I don’t believe she was formally an “acting.” Nor did she wish to be, according to many conversations she held on the subject with staff and friends. Similarly, ECA assistant secretary Pat Harrison held the under secretary’s authorities for several months before the arrival of Karen Hughes (and Harrison’s replacement at ECA, Dina Powell, who was formally named “deputy under secretary — the first time this was done, as far as I know). In any case, the point of your excellent chart is to show incumbency of formally nominated and Senate-confirmed under secretaries — not the many good folks who have filled in during the long gaps in tenure. (By the way, the first version of your chart that I saw was prepared a couple of years ago by Ambassador Brian Carlson and posted on Matt Armstrong’s now dormant blog, and I believe Matt has updated it periodically. You may wish to check it out and confirm that your data coincide.)