The easiest and quickest way would be to reorganize the data as in:

    Means
1    12.1
1    13.1
1    14.1
1    12.1
   
2    15.2
2    16.3
2    18.2
2    11.2
2    13.3
   
3    12.1
3    13.1
3    16.3
3    18.2
3    11.2

Once done, on the chart, double-click the plotted column.  In the
resulting dialog box, from the Options tab, set the Gap Width to zero.

And, no, you don't have to reorganize your original dataset.  Use a
copy.