It may depend a little on the design of your dimensions / data model. I assume that you are just using two fields in a pivot table / line chart, one dimension pivoted to the top, that's what you call columns then, right?
You could probably just use
for your conditionally show expression, replace FIELDNAME with your field name you used for the columns /series dimension.
Hope this helps,
Thank you Stefan,
I found a solution.
To fix ideas, for example, a pivot table with 4 dimensions for the columns, X1, X2, X3, X4 and 1 for the rows, X5. This configuration can give me a table with 0 or several columns : it depends on current selection on these or other fields.
In the "layout - show - conditional" property of the chart, I use the condition "count( distinct X1& '$' & X2 & '$' & X3& '$' & X4) = 1", where $ is a separator not used in the values of the four dimensions. This solution is not perfect but it is good for my problem (not perfect, because, I have to "freeze" the pivot to have all time the same fields in column).
I consider that the solution is found but I would be happy if a more elegant solution was found...