Incorrect BTS APY sometimes shown on #/pools/[pool_id] page

[christophersanborn]

Anecdotally, may be related to when pool history includes deposits.

Scenario: Pool ARTCASA.BTSMM pairs BTS with ARTCASA. On Dec 9 I purchased ARTCASA out of the pool and re-deposited with equivalent BTS, over multiple iterations. There was also some bot activity that sold ARTCASA back into the pool, but overall far more was purchased out of the pool than traded back into the pool. Purchasing ARTCASA out of the pool raises the price of ARTCASA against BTS, and thereby raises the withdraw value of ARTCASA.BTSMM against BTS.

If APY is computed as an increase in value of the pool share token against BTS, this should result in a net positive APY value. However, during this day the APY was reported as a negative value, specifically: -100%.

On the other hand, If the APY is computed not by looking at the withdraw value against BTS, but rather as the square-root growth of Pool K (k=xy) divided over the number of outstanding shares, then again, this should have shown a positive value, as the pool grew K by retaining taker fees on exchanges. (In fact, if growth is computed in this latter manner, then APY should never be negative.)

The negative APY value causes the pool to be incorrectly listed at the bottom of the APY rankings.