\legend{
    $\begin{aligned}[t]
    yy&=x^2\\[-2pt]
    y&=x\\[-2pt]
    yyy&=4-x\\[0pt]
    zzz&=y+x\\[0pt]
    k&{}\\[-2pt]
     zzz&=y+x
    \end{aligned}$,
    \strut,\strut,\strut,\strut,\strut % second legend entry
  }