I may be misinterpreting your query, but if you select to 'show all values' rather than exclude when no selection is made then you will get the chart for all data instead of filtered to the selected area.
thanks for your answer. The problem here is that I am not using the raw data but the aggregated ones and the overall mean is different from the mean of the means in each region... so if I choose to show all vaues, I would get the sum or average of my values by region instead of the overall mean that I have already calculated...
Ah yes, that is an issue. Unless you also have a field in your data which shows the number of records that were averaged to get the regional mean, then you can't retrieve the information you need to get the true mean, only the mean-of-means, since you've lost information in the averaging. If you can get the count, then you can just just sum the product of the regional means by the regional counts and then divide by the number of records being displayed via the filter. But without that count, not enough information is left to derive the true value.