:-) Even folks who have been researching ENSO for decades are learning something new all the time, so don't worry.  Your diagram is one way to examine ENSO variability, but I find these links to be an easier way to examine historical ENSO fluctuations-- and they go back to 1950:

http://www.cpc.ncep.noaa.gov/products/analysis_monitoring/ensostuff/ens…

http://www.cpc.ncep.noaa.gov/products/analysis_monitoring/lanina/enso_e…  (see slide #21  for time series of ONI/Nino-3.4 index)

What we notice from looking at these time series is that stronger El Ninos are often followed by La Nina the next year.  There are fairly well-founded physical mechanisms (recharge-discharge) that explain why this is probably the case despite the small number of "strong" El Ninos in our historical record.  However, the inverse is less likely.  That is, El Ninos (strong or not) are not often preceeded by La Nina (in the previous year).