title: What does the `fontsize` represent in `grid` graphics?
body: <p>The <code>fontsize</code> is used to determine the size of text or points in R graphics. But what does it actually represent? For example,</p>
  <pre><code>grid::grid.newpage()
  grid::grid.points(default.units = &quot;npc&quot;, 
                    gp = gpar(fontsize = 100))
  </code></pre>
  <p><a href="https://i.stack.imgur.com/zAEjb.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/zAEjb.png" alt="enter image description here" /></a></p>
  <p>Here, the point <code>fontsize</code> is 100. So, I have two questions:</p>
  <ol>
  <li>What does the value mean, the radius, the diameter or the area of a point? Can I find any reference?</li>
  <li>What is the unit of 100? In specific, does it mean 100 pointsize (1 fontsize = 1pt)? Otherwise, is it determined by device-specification? If so, 1 fontsize = ?pt.</li>
  </ol>
  <p>Also, I have another question that, in language R, is there any handy functions to convert <code>pt</code> to pixel (<code>px</code>)? For example, we can convert a <code>pt</code> unit to a <code>mm</code> unit as in</p>
  <pre><code>convertUnit(unit(1, &quot;pt&quot;), &quot;mm&quot;, valueOnly = TRUE)
  </code></pre>
  <p>So, is there any function to query the dimension of the screen and then do the transformation from <code>pt</code> to <code>px</code>?</p>
  <p>Thanks!</p>