You can use if like = If(A=a,ValueToShow) like that.
If u wish to give user a control to search for values then U can make it by
Chart Properties -> Presentation Select field to search for & select Dropdown select
& dropdown will appear near the dimension & user can search for the value by clicking on dropdown & typing the value to search for.
when you say that columnA= a text value, you could also simply use a list box to hold all the values in ColumnA and let the user select and search (and see excluded values etc). This uses the basic tenets of the QlikView association and selection model. If there is no calculation to do might be an easier set up and then just have column A as the dimension in your chart and sum(ColumnB) [or appropriate operation] to give the value.