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I am trying to calculate monthly means by year of phosphates and nitrates from a multi year data set. Can anybody help me out with the most effective way to do this? My data looks like this: Collection_Date                   Test.Name              Value 2000-01-24 17:00:00            Phosphate               0.108 2000-01-24 17:00:00            Nitrate                     0.037 2001-11-12 08:45:00            Phosphate               0.45 ... Thanks and sorry for the blatantly "newbie" que...

Hi all, I have scoured the archives of this forum but nothing quite seems to fit the bill... I would like to plot a graph displaying two variables (y axes) that share date as the x axis. However, the date values for each variable are not the same - for example, some parasitoids were not released on days that collections from the trap took place, whilst sometimes releases did occur on the same