To the best of my knowledge, constructing a three-dimensional Herzprung-Russell (H-R) diagram where the third axis represents elements by maximum atomic number or weight is not a commonly used approach in astrophysics. The H-R diagram is primarily a two-dimensional plot that shows the relationship between the luminosity (or absolute magnitude) and the surface temperature (or spectral type) of stars.
The H-R diagram is a valuable tool for studying stellar evolution and classifying stars based on their intrinsic properties. It provides insights into the various stages of stellar life cycles, such as main sequence stars, red giants, white dwarfs, and more. However, the H-R diagram does not typically incorporate elemental information.
That being said, there have been efforts to incorporate additional dimensions into the H-R diagram to represent other stellar parameters, such as metallicity (the abundance of elements heavier than helium) or stellar age. These variations allow astronomers to study the influence of these parameters on stellar evolution and properties.
While it is possible to imagine extending the H-R diagram into three dimensions to include an axis representing elements by atomic number or weight, this is not a standard or widely adopted approach in astrophysics. The H-R diagram's traditional representation has proven to be a powerful and effective tool for understanding stellar characteristics and evolution, and adding a third dimension for elements might introduce significant complexity without necessarily providing substantial new insights.