 Formula interface</span>
<span class='va'>tree2</span> <span class='op'>&lt;-</span> <span class='fu'>networktree</span><span class='op'>(</span><span class='va'>y1</span> <span class='op'>+</span> <span class='va'>y2</span> <span class='op'>+</span> <span class='va'>y3</span> <span class='op'>~</span> <span class='va'>trend</span> <span class='op'>+</span> <span class='va'>foo</span>, data<span class='op'>=</span><span class='va'>d</span><span class='op'>)</span>

<span class='co'>## plot </span>
<span cl