Clicking ×N sometimes flew to a blank area far below the tree. Cause:
family-chart's cardToMiddle scales datum.x by the zoom factor k but not datum.y
(`y = height/2 - datum.y`, missing the ·k), so vertical centering is only
correct at k=1 and drifts by datum.y·(k−1) at any other zoom — worse the deeper
the person sits. That's why it worked only when the view happened to be near 1:1.
Compensate by pre-multiplying the y we pass to cardToMiddle by the current
scale, cancelling the library's missing ·k. x was already correct.
Signed-off-by: Justin Paul <justin@jpaul.me>
justin
bf1576252b