Problem with field parameter and empty slicer

Hi PBI_VL , You cannot dynamically return a column directly from a measure, so use the disconnected table as the axis and then use TREATAS inside the measure to apply the correct column filter. For example, create a disconnected table that contains all category values from both columns, like:
AxisTable =
DISTINCT(
UNION(
SELECTCOLUMNS(Coverages, "Category", Coverages[AGE_CATEGORY]),
SELECTCOLUMNS(Coverages, "Category", Coverages[COVER_AGE_CATEGORY])
)
)

Then use AxisTable[Category] on the Y axis and create a measure like:
ClientCount Dynamic =
VAR CoverSelected =
ISFILTERED(DimCover[Cover])
RETURN
IF(
CoverSelected,
CALCULATE(
[ClientCount],
TREATAS(VALUES(AxisTable[Category]), Coverages[COVER_AGE_CATEGORY])
),
CALCULATE(
[ClientCount],
TREATAS(VALUES(AxisTable[Category]), Coverages[AGE_CATEGORY])
)
)

With this setup, when no Cover is selected the axis behaves like AGE_CATEGORY and when a Cover is selected it behaves like COVER_AGE_CATEGORY. You also avoid the unstable behaviour caused by the field parameter.