This is probably the result of a Dashboard change which do not provide you the correct result. Because the Block used column is hidden you cannot see it as a result or as it is disabled in the chart Data Source -> Y-Axis Source button. (Probably it was set up, before you hide the column.) If you want to see only the PCT Used in the chart, you need to make the column Block used visible in the object Column Configuration. For that select it and add it to the display view. Then go into Chart View -> Chart Data Source button -> Y-Axis Source selection and de-select the Block Used column. Reload the object and save the dashboard. From this moment on, you should see just the PCT Used data in the chart.