t of the perspective plot. The default is <code>"grey"</code>.</p></li>
<li><p><code>bt.persp_shade</code>: A ratio defining the shade of the
      surface. The default is <code>0.35</code>.</p></li>
<li><p><code>bt.persp_{theta, phi}</code>: Angles (in degree) defining the
      viewing direction of the perspective plot. <code>theta</code> corresponds to
      the azimuthal direction (default: <code>330</code>) and <code>phi</code> to the
      colatitude (default: <code>15</code>).</p></li>
<li><p><code>b