First, I am not posting this to make any statement about Paul Krugman, or anything like that.  But when comparing time series data, choosing the starting point can be more imporant than it seems, as this blogger demonstrates.  You could imagine investing strategies being compared in a similar fashion.  One graph shows strategy A is better, but with the same data and a different start date, another graph would show that strategy B is better. 

Another issue is the choice of the other data sets – they can also be chosen to look “fair” but those time series contain special features that make the analysis less than fair.  For example, compare company X with specially chosen company Y, because of the special charges for Y in a particular quarter make some observation about X seem more true.

Simple comparisons are not always so simple.