In agreement with requests from designers Xschem will not store dB and phase values for nodes in AC graphs, but will just store magnitude and phase. So if you need the value in dB for voltage

xxx

you need to replace

xxx

in your old AC graphs with (including quotes):

"xxx log20()"

or

"Label; xxx log20()"

(if you want a custom Label naming the waveform). Just for the curious, the

"xxx db20()"

expression will calculate

20*log10(xxx)