Comparing EPL efficiency across years

Last week I posted a graph which showed the Offensive and Defensive efficiency of teams across Europe's four major leagues.  The chart was measured teams' conversion rates against the number of shots they faced before conceding a goal.  As with all statistics, this is informative in isolation, but doesn't provide a full understanding of the situation without further context - as in,without further information we can't say if Liverpool's offensive profligacy is a one-season trend due solely to Luis Suarez's left boot, Andy Carroll's relative lack of presence or even if Steven Gerrard's long-term absence contributes to such a statistical  malaise.

In order to answer these questions, we need to compare this year to others.

The following charts show first the change in Offensive/Defensive efficiency for each team in the 2010-11 and 2011-12 Premier League, and secondly the change over the last three years (when shots/shots faced statistics became more readily available).  In the first graph, lines chart the year-on-year change for each club.  Such lines aren't present in the second chart as they would make the chart practically unintelligible.  Enjoy!

I  highly recommend clicking each image to enlarge it.