Graphic of expression on BrainSeq, what is on Y?

Question

[philosophical.raptor111]

I dont understand what units are on Y scale. Expression cannot be negative, does it? What units shows Y then? https://ibb.co/KKfSZnV

[i.sudbery]

The BrainSeq website doesn't provide enough information to tell, you'd have to read the paper. My guess would be either log TPM or log FPKM, which can be negative if the TPM or FPKM is less than 1.