Bars like those are usually range markers - the highest and lowest points in the data being marked by the, in this case, 2.4 and .6. If you want to show those, click on a column in your graph, go to Format Data Series, Go to the Y Error Bars tab and click Both. If you make it a percentage and set that value to 50, it will show you a bar going down and stopping at the 50% and going up and stopping 50% above the data column.
I just find it easier, however, to calculate the mean next to the original data and include that column of data in the graph also.