Or, you could try this. Let's assume that you data is in rows, with column A showing the month (and possibly year), and column B has the data you are displaying in a column graph.
First, go through your data and segregate the data by year. So, year 1 would be in column B, year 2 would be in column C, year 3 in column D, and year 4 in column E.
Set up the graph to take data from columns A through E.
Add a trend line to each data series.
Adjust the overlap and column width of each series of columns, so that you have 100% overlap (meaning that the bar will appear in the same spot for each month-wide bin), then fatten the columns up as desired.