Your raster is not an integer raster, it's probably a float 32. The graph tool will not work with such rasters in that format.
You can employ a trick and multiply the raster by 1 million to move the decimal point 6 places to the right. Pass that output through the int() tool to ensure it becomes an integer raster. You will now be able to create your histogram. Just be aware that you would need to divide the values by 1 million to return them to the true NDVI value.
This approach works well if you want to just look at the data. If you want to create a quality figure for a report then you might need to do what @MichaelStimson suggests as you will have more control over the final design of the graph.