Hi Norbert, Try doing this in addition to the steps that you've already taken. Once you've defined a KPI for a given cell:
d. Click "Examine" (top right, next to designer)
e. Select the cell, and then select a bullet chart from the drop down (inside the examine box). A colored number line should appear.
f. Click the down arrow in the top right area inside the Examine box and select "copy to story."
Feel free to mark my comment as correct answer if it resolves your issue.
Mohammad
P.S. sometimes the number line doesn't work due to bugs in BO Cloud.