 with (1,0) perform Math.atan(segVectorY/segVectorX).</div>
<dl>
<dt><span class="paramLabel">Parameters:</span></dt>
<dd><code>index</code> - the curve index specifying the curve to analyse</dd>
<dd><code>distance</code> - the distance from start to end of curve (0.0...1.0)</dd>
<dt><span class="returnLabel">Returns:</span></dt>
<dd>a unit vector at the specified point on the curve represented
                as a line, parallel with the curve. If the distance or curve is
                invalid, <code>mxC