The type of diagrams that illustrate the relationship between two variables such as height and weight or the hardness of a ball bearing and its diameter are:

Scatter diagrams.

Explanation:

Scatter diagrams more popular by the name of the X-Y graph are useful mathematical methods for analyzing the relationship between two variables .  as the name implies, it is a type of a paper sheet on which the detail points referring to the interest variables are spaced. The scatter diagram displays pairs of numeric values to seek a correlation between them, with one variable on each axis. If there is a connection between the variables, the points may fall into a line or curve. The better the connection, the more close the dots hug the line.

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