Postgres 11

By using db<>fiddle, you agree to license everything you submit by Creative Commons CC0.

select * 
from crosstab(
	$$
	select 
		unixdatetime, 
		gaugesummaryid, 
		value::double precision 
	from 
	(values
		(1546300800,187923,1.5),
		(1546387200,187923,1.5),
		(1546473600,187923,1.5),
		(1546560000,187923,1.75),
		(1546646400,187923,1.75),
		(1546732800,187923,1.75),
		(1546819200,187923,1.75),
		(1546905600,187923,1.5),
		(1546992000,187923,1.5),
		(1547078400,187923,1.5),
		(1547164800,187923,1.5),
		(1547337600,187924,200),
		(1547424000,187924,200),
		(1547510400,187924,200),
		(1547596800,187924,200),
		(1547683200,187924,200),
		(1547769600,187924,200),
		(1547856000,187924,200),
		(1547942400,187924,200),
		(1548028800,187924,200),
		(1548115200,187924,200),
		(1548201600,187924,200),
		(1548288000,187924,200),
		(1546300800,187926,120),
		(1546387200,187926,120),
		(1546473600,187926,120),
		(1546560000,187926,110),
		(1546646400,187926,110),
		(1546732800,187926,110),
		(1546819200,187926,110),
		(1546905600,187926,115),
		(1546992000,187926,115),
		(1547078400,187926,115),
		(1547942400,187927,100),
		(1548028800,187927,100),
		(1548115200,187927,100),
		(1548201600,187927,100),
		(1548288000,187927,100)
	) as t (unixdatetime, gaugesummaryid, value)
	order by unixdatetime
	$$
	) as final_result (
		unixdatetime int, 
        trace1 double precision, 
        trace2 double precision, 
        trace3 double precision, 
        trace4 double precision
		)

unixdatetime	trace1	trace2	trace3	trace4
1546300800	120	1.5	null	null
1546387200	120	1.5	null	null
1546473600	120	1.5	null	null
1546560000	110	1.75	null	null
1546646400	1.75	110	null	null
1546732800	1.75	110	null	null
1546819200	110	1.75	null	null
1546905600	115	1.5	null	null
1546992000	115	1.5	null	null
1547078400	1.5	115	null	null
1547164800	1.5	null	null	null
1547337600	200	null	null	null
1547424000	200	null	null	null
1547510400	200	null	null	null
1547596800	200	null	null	null
1547683200	200	null	null	null
1547769600	200	null	null	null
1547856000	200	null	null	null
1547942400	100	200	null	null
1548028800	200	100	null	null
1548115200	200	100	null	null
1548201600	200	100	null	null
1548288000	100	200	null	null