The $y$ variable is the modules of natural
rubber, $x_1$ is the amount of
decomposed dicumyl peroxide, and $x_2$ is the temperature.
Figures (a) and (b) display a typical linear pattern by slicing
on $x_1$ and visuaalizing the corresponding distribution of $y$
on $x_2$.
The parametric model
$$E(y|x)=c_1x_1x_2 + c_2x_1 + c_3 x_2 + c_4 $$
suggested there is unsatisfactory in view
of the obvious line patterns found in
the residual plot, Figure (c).