Geometric multipliers for $\min \frac{1}{2} (x_1^2 +x_2^2) ~~ s.t. ~ x_1\leq1$

Question

I am reading about geometric multipliers and I am trying to understand an example from here [1]. The visualization of this example is given below. According to the text, there is a unique geometric multiplier $\mu^*=1$. Could you please someone explain how the shaded part of the figure is drawn and how the solution is computed? Shall we draw the problem in three dimensions?

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^{[1] Nonlinear Programming, Bertsekas.}