The Trend Analysis tool has an option to plot "Effective VO2max" and "VO2max estimate". Sorry if this has been answered before, but I could not find any info clearly explaining the difference between the two.
Depending on the browser you're using, the 'VO2max estimate' should be part of a group labeled 'FIT details'. Those values come directly from the *.fit files (i.e. calculated by Garmin or rather Firstbeat), while 'Effective VO2max' is the value calculated by Runalyze.