Here's why:

* Luminosity: Giant and supergiant stars are incredibly luminous, meaning they emit a large amount of energy. This places them high on the Y-axis of the H-R diagram, which represents luminosity.

* Temperature: While they are very luminous, giant and supergiant stars are relatively cool compared to main sequence stars. They have expanded significantly, decreasing their surface temperature. This places them to the right on the X-axis of the H-R diagram, which represents temperature.

Key points:

* The exact position of a giant or supergiant star on the H-R diagram depends on its specific mass, age, and evolutionary stage.

* There are different types of giant and supergiant stars (red giants, blue giants, etc.), each with their own characteristics and position on the diagram.

Overall, the upper right region of the H-R diagram is where you'll find the most evolved stars, those that have exhausted their hydrogen fuel and have expanded dramatically.