Display (tmax - tmin) in secondary axis of plot

[baqwas]

Newbie here, please.

How to create a data series which is the difference between the daily 'tmax' and 'tmin" and then plot it on the secondary axis of a daily plot? Unsure about appending another column to the data vector (as in data.fetch()). Should be able to work out the secondary axis on my own once I get the guidance on the new column (i.e. 'tdiff' for example). Thanks for your understanding.

Regards.