That is a good drawing. I like the exaggerated slope.
The lines should be sloped as much as practical to assure water is actually drained. I have seen specs of 1/4" per foot which can lead to water build up like previous poster said.
I prefer to slope line to compressor to assure that OIL as well as water drains into compressor tank.
One problem I saw with the drawings is the first leg from compressor outlet is not sloped and the pressure gauge (PI) is susceptible to flooding. A flooded PI will surely get freeze damaged if ambient temp drops below freezing. Must mount PI above water level.