315 votesJohn Barr commented
I like this idea, and Tim's idea of how it would work is the same as what I would want to have on a stock graph. It might become a little more complicated for stocks where there are multiple positions i.e. where you purchase the stock originally and then buy more when each dividend is declared as in a DRIP situation. Perhaps you would have a button (like the confirms calls button on the bottom of the graph which would toggle the feature on and off. Another way to do it might be a crosshair cursor approach where the vertical line is the purchase date (or first purchase date) and the horizontal is the average cost basis per share.