After further investigation it looks like the way you implemented your stacked colimns to work in 2.6.1 was implemented incorrectly but visually provided what you wanted.
This issue has been fixed in our latest version therefore it is preventing the visual column chart you want.
When stacking bars or columns they are programmed to stack the entire bar/column value on top of the one it's stacked on.
For example, if the X-value Broccoli had a Y-value of 4 in the first series and 5 in the second series. The second series will start at the stacked value of 4 and add 5 to it.
Meaning the column for datapoint with X-Value Broccoli in second stacked series will start at 4 and end on 9.
If you wish the second series to end on a specific Y-Value you must take all the previous Y-values from the datapoints in the series that it is stacked on top of into account.
For example, if you wish the second series for the Broccoli X-Value to end on 7 rather than 9, you must pass 3 as the datapoint Y-value rather than 5.
Let me know if you have any questions.