1) from the visual it seems to me that the starting year is the most relevant. If you start in a good year, it will mostly turn out right, regardless whenever your end (exceptions aside, for which see point 2). If you start in a bad year it will mostly work out badly unless you really have some time to spare or manage to run into a very rare occasion (e.g. starting in 1947 and ending in the mid 1950's). But that's just from the visual, which can be very misleading, so the raw data points would be interesting to do some statistic exercises. If that holds true though, it could be a good guideline - assess the current returns of a particular fund and do not invest [in it] if the current returns are not high enough. While this would make you, by definition, miss out on any really spectacular returns, it could reduce risk enormously without sacrificing much in terms of returns.
2) if you happen to have invested in a fund that took a nose-dive, hang on to it and don't sell for a long while, as in the long run you're apparently very likely to end up at the 20-year median (guess it's called a median for a reason ;-) which is not too bad. At the very least your loss is going to be minimized with time.