{point_id} — current value of a raw or virtual point
delta({point_id}) — change since last poll (can be negative)
pos_delta({point_id}) — change since last poll, clamped to ≥ 0
reset_delta({point_id}) — change since last poll; if the counter went
backwards (rollover/reset), uses the current value instead of a negative delta
dt — seconds since this app's last poll
dt({point_id}) — seconds since that point's own last reading changed
Operators: + - * / and parentheses. A point can reference its own Point
ID for running totals — its previous value is read from stored history automatically.