Man, that is one ill-explained figure. What is 0? Change as compared to last year? If this is the case, then the graph illustrates acceleration in temperature (which kinda makes sense), but it does not make 1934 the warmest year. Suppose 1933 had an average temperature of 10 degrees Celsius. If 1934 had an average temperature of 11 C, 1934 would be marked as a 1 on the figure. However, if 1997 had an average temperature 30 C, and 1998 had 30.5 C, 1998 would be 0.5 on the figure.
It's late and I didn't spend a lot of time on this, so please correct me if I'm wrong here.