title: Mixed horizontal and vertical alligned use case nodes
body: <p>I want to display nodes in a Use Case diagram more compact than Graphviz does by default. Say for example the following cases are wrapped in a rectangle:</p>
  
  <pre><code>actor A
  actor B
  A - (case a)
  (case b) - B
  A - (mixed case)
  (mixed case) - B
  </code></pre>
  
  <p>By default the cases are aligned vertically. I'd prefer to have (case a) an (case b) side-by-side and (mixed case) centred and below the previous cases. I've also tried to use -[hidden]- links</p>
  
  <pre><code>(case a) -[hidden]- (case b)
  </code></pre>
  
  <p>but then (mixed case) gets left aligned and cases a and b are too wide apart. Here's how it looks in full beauty:</p>
  
  <pre><code>@startuml
  left to right direction
  Source  &lt;&lt;Operator&gt;&gt;
  Sink    &lt;&lt;Operator&gt;&gt;
  Source ..&gt; Sink : notify service address
  rectangle "Init phase" {
      Source -&gt; (Prepare service)
      (Prepare service) -[hidden]-(Register with service)
      (Register with service) &lt;- Sink
      Source -&gt; (Secure channel) : &lt;&lt;initiate&gt;&gt;
      (Secure channel) &lt;-- Sink : &lt;&lt;participate&gt;&gt;
      Source --&gt; (Ensure readiness) : &lt;&lt;participate&gt;&gt;
      (Ensure readiness) &lt;- Sink : &lt;&lt;initiate&gt;&gt;
  }
  @enduml
  </code></pre>
  
  <p>Image here:
  <img src="https://i.stack.imgur.com/2xrDg.png" alt="UC format example"></p>