Starting in V15, you can choose not to display disabled measurements in graph legends.
For example, see the graph below. Two of the measurements are disabled that show grayed out in the measurement legend.
To change this behavior, select Options > Environment Options from the menu and click on the Project tab. Find the option Disabled measurements in legends option and make sure it is unchecked.
Notice how the graph looks after his option is changed. You might need to simulate to get the display to update.
When disabled measurements are not displayed in the legend, use the project browser to edit or enable them.
Because this option is an environment option, it applies to all projects you open, not just the current project.