It’s an image. But your perception of it turns it into a personalized “graph”*. That’s the whole point. But call it a chart if it makes you feel better.
And it’s still works even if on a monitor. If you don’t see peaks at the right end, whether due to the monitor or your own eyes, then the results are still meaningful. It means you are limited by your eyes or your monitor.
*A graph is only a curved line one of several mathematical definitions of graph. The statistics definition: A graph is a picture that represents data in an organized manner. And graph theory doesn’t concern lines, even though it uses them to represent graphs.
