well, compare it to meteor swarm, it does the same damage, but with a 40 foot radius, so lets do the math, (pi)40^2, or about 5000 ft^2, or 960 ft by 5 ft, or about 4800 ft^2, so slightly less area for more versatility, as well as the fact that if you choose to roll to hit with the lightning, you can do 26d6, while meteor swarm can do 32d6. so in the end, its (slightly) less area, and less damage, for more versatility in how it's spread out, so consider that.
was not sure how to add the wording for all the branching lines to add the piercing damage...
also, this is a 9th level spell, this is a slot that a twined empowered fire ball. 27d6 twice... or hell a widened maximized fire ball, for 108 damage in a 40ft radius.
Avatar by Szilard, thank you sir for the fine work!