You have the W’bal drop selected for the plot on the fitness chart.
If you go to activities list view and add the column W’bal drop, do you have any numbers in that column? Those are the numbers that should display on the graph you set up.
If you’ve never been above FTP, there has never been a W’ drop and the plot remains at zero. It is very unlikely that this is the case but you may have a FTP setting that is completely off??