Use vertical separators when plotting data for irregular intervals
Say you have to plot monthly footfall at a store for a pretty irregular interval, Aug 2010 to Apr 2011. Users are typically used to seeing monthly data for 12 months one after another or quarterly data for the four quarters, but surely not data from Aug 2010 to Apr 2011.
Typically, the dynamics from one calendar year to another change a lot. To make sure that the user knows when 2010 ends and when 2011 starts, even when he is deeply immersed in his data analysis, put a vertical separator line between 2010 and 2011, as shown in the following screenshot: