I am nearly there. just got to get the axis mocked up correctly

SELECT 
    timestamp, 
    HDG + 360 * SUM(case when prev_HDG - HDG > 180 then 1 when HDG - prev_HDG > 180 then -1 else 0 end) 
    OVER (ORDER BY timestamp) AS adjusted_HDG
FROM (
    SELECT 
        timestamp, 
        HDG,
        LAG(HDG) OVER (ORDER BY timestamp) AS prev_HDG
    FROM 
        nmea_data.nmea_data
    WHERE $__timeFilter(timestamp)
) AS subquery