The great color theorist Albert Munsell observed that a given color reaches its greatest chroma at one particular value, called the home value or the peak saturation value. That peak value varies from color to color. An orange-yellow, for example, is most intense at a very light value, while blue is strongest when it is very dark. Red reaches maximum chroma at a middle value.
The hand-painted chart takes the three hues through all possible degrees of chroma and value. Yellow becomes a brown when it darkens, and you just can't get a strong chroma from a dark yellow.
In the chart, chroma is meant to be constant along a vertical line, while value is constant on a horizontal line. It’s good color mixing practice to make these charts, and Munsell-schooled artists benefit greatly from the experience.
For these charts I have arbitrarily limited the chroma range to six steps. But in Munsell’s system, there can be many more steps, and theoretically there’s no upper limit to the measurement of chroma. New pigments keep pushing the range farther out.
This digital chart from David Briggs's excellent website huevaluechroma shows a range of hues taken through the paces. Follow that link to his discussion of the same topic.