On the H-R diagram, lines of constant radius slants down from left to right.

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Lines of constant radius on the H-R diagram are called "evolutionary tracks" or "isochrones" and they slant down from left to right. This is because as a star evolves, it starts off on the left side of the diagram as a hot, luminous star with a large radius, and as it ages and its core contracts, it moves downward and to the right, becoming cooler, less luminous, and smaller in radius. Therefore, lines of constant radius have to slope downward to account for this evolution.

On the H-R Diagram, Lines of Constant Radius Slants Down