In my opinion, it may be to reflect the healing nature of the user's HP, after the attack. Think about it,
Turn 1: Here HP is max (the beginning of the curve)
Your Helioptile is attacked
End of turn 1: HP goes down, as is symbolised by the down ward curve in a parabola
Turn 2: Helioptile uses Parabolic Charge, and Regains its health, thus pushing the HP upwards. This is again shown by the way the parabola moves upwards again.
Again, just mere speculation from my side, and the may be totally different.