Hi Onno,
So, the vertical axis is dBi.
-10dBi means that the EIRP level in that direction is 10dB lower than with an isotropical radiator
That's OK.
and probably about 15dB lower than the level in the direction of maximum/peak gain
Why do you say 15 dB lower than the maximum/peak gain? Why do you take the number of 15? In the specs I see this AP-315 has a maximum peak gain of 3.6 dBi in 2.4 GHz, so it would be 13.6 dB lower than the maximum/peak gain. Are you rounding the value?
Regards,
Julián