How to sum and average several data points

I would like to know how I can add and average several data points, that is, I want to average several temperatures and show them in a gauge as the general temperature of the building
Thank you very much 
Hi maikel8a,
If you don't want a history of it, you could select all the points with an mapointquery, get their values with an mapointvalues and get the average with a
<macalc input="(points  maSum:'value') / points.length" output="pointAverage"></macalc>
But, if you would like a history you should add these points into the context of a meta point, have it update on a cron (or one point from each data source) and compute the average there. That will have the advantage of being simply usable however any other data point would be.

Thank you very much @phildunlap, I will try to prove with this what you tell me. What I need is just to show it.
Thanks again