add multiple x values to multiple instances of y coordinate
 

I have days of the month on the Y axis and a set of other numbers on the x axis. If I have 2 cells which are both labeled August 15th (for example), I want the data that corresponds to this entry to be added together along the x axis on the chart. Right now, the two sets of data do both show up on the August 15th bar, but subdivided within a single bar rather than being added together. Is there a way to do this?
Thanks

for example:
A, B
aug 15th, 8
aug 15th, 10

I want the bar to show 18 rather than the bar being subdivided to show both the 8 and 10 and only going up to 10 on the x axis