<p>I agree with @whuber's answer, but another issue is that you are implying multiple questions in a single post: </p>

<ol>
<li>What does <span class="math-container">$\mathbb{E}||\hat{\beta}(k) - \beta||^2$</span> in the <em>y</em>-axis of fig. <span class="math-container">$3.6$</span>  of Elements of Statistical Learning mean?</li>
<li>Is my interpretation of how fig. <span class="math-container">$3.6$</span>  of Elements of Statistical Learning was made correct?</li>
</ol>

<p>And ultimately:</p>

<ol start="3">
<li>Why does my implementation of fig. <span class="math-container">$3.6$</span> of Elements of Statistical Learning not result in the same plot?</li>
</ol>

<p>On their own, (1) and (2) are both perfectly on-topic. Once you have the answers to these, (3) might be better suited for SO. This might seem like a hassle, but to understand why a compound question is a problem, keep the goal of SE in mind: Ultimately your post should be useful to future readers with similar issues.</p>

<ul>
<li>If someone else also wants to know (1), should their question be marked a duplicate of yours? Should they have to find the relevant piece of information in the context of a larger question? Or the other way around, crippling your ability to gain an answer to (2) and (3)? </li>
<li>What should the title of your question be, such that others with any of your questions can find it easily?</li>
<li>If a reader knows the answer to (1), or (2), or (3), but cannot/does not want to answer the other questions, is it okay to have an answer that only partially addresses the question?</li>
</ul>

<p>There is no limit to the number of questions you can ask here and it is both much easier for others with <em>questions</em> and for anyone interested in <em>answering</em> if you post your questions separately. Plus, you can get more reputation from multiple on-topic questions, perhaps allowing you to put a bounty on a question that you want to receive more attention.</p>
