LTspice, plotting a node voltage problem
Like Michael Karas already answered and LTspice also states in its help file:


  Each node in the circuit requires a unique name.


Still, playing around with labels on the same node seems to reveal that


LTspice prefers the bottom labels above the top labels in a schematic (first rule)
next, LTspice prefers right side labels above left side labels (second rule)
LTspice picks the label that fits above rules best and uses that name as node name
order of placement or alphanumerical order of naming is not relevant
(what OP also already discovered) you can still use the other (less preferred) label name to refer to its node (in the waveform viewer as well as in behavioral sources, etc)


So, in your case -9v is lower than pnp_c, so -9v 'wins' becoming the final node name.  

And npn_c is lower than +9v (first rule, despite it is more to the right side (second rule)) , so npn_c 'wins' becoming the final node name.  

The ugly way to solve your issue is swapping -9v and pnp_c in vertical order.
A better way is not connecting several labels to the same node, but insert a component between the labels instead.
I found that inserting a jumper will not show 2 different node names (despite the description suggests).
Inserting an e.g. 1m&Omega; resistor between the labels -9v and pnp_c would work (same applies to other labels on the same net).