Have a look in the relevant skin.conf under [ImageGenerator] and then look at 
the aggregate_type and aggregate_interval config options under each [[ ]] 
stanza. By default the plots for week and longer periods plot an average (set 
by aggregate_type config option) calculated over the aggregate_interval period. 
For week this is a one hour average, month is a three hour average and year is 
a one day average (day plots plots do no averaging). Consequently week, month 
and year plots of data such as UV and radiation can appear to display low 
values.

One of the main reasons for this approach is to avoid excessive processor 
loads/times when generating large plots. For a 5 minute archive period one day 
consists of 288 data points, for a one year plot this would be over 100 000 
data points if there was not some sort of aggregation being used. As mentioned 
you can change aggregate_type to plot the maximum value if you want a more 
realistic plot. Have a read of The image generator section in the Customization 
Guide (http://weewx.com/docs/customizing.htm#image_generator)

Gary

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