QGIS very slow and stops when increasing points size / number

Question

I used the "graduated" style option to represent data by year.

Then for each year I used this method with a custom function to increase point size according to the number of points at the same location. increasing marker...

It works well but QGIS takes more than 30 minutes to display and the same time to save the map using composer. It often just stops.

Is there another way, faster, to get this result?

I'm using QGIS 2.12.

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Here a part of what I would like to obtain (using increasing markers size...)

Here what I obtained with csk suggestion : new field creation, convex hull and centroid extraction something is probably wrong.

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Finally, I used this method which allow to draw points with size relative to the frequency at one location. It is very fast and uses both fied calculator as suggested, but a categorized symbology based on the new field. Increasing marker size based on frequency...

Answer 1

This method should work, but I wasn't able to test it because QGIS won't work properly for me today (my computer is in a bad mood, I guess). Let me know if you run into any issues and I'll try to address them.

1. Create a new field, "date_range", which has the starting date (or the ending date, or the date in the middle of the range, whichever makes more sense to you) of the date range for each category. Use the Field Calculator and an expression like:

   CASE 
   WHEN "datefield">=1841 AND "datefield"<1870 THEN 1841
   WHEN "datefield">=1870 AND "datefield"<1914 THEN 1870
   (etc.)
   END
   

2. Create a convex hull for the points in each "date_range", using the minimum bounding geometry algorithm (Processing toolbox).

3. Extract the centroids of the convex hulls
4. Symbolize the centroids layer in a graduated symbology based on the "date_range" field. Use data-defined settings to control the symbol size based on how many points went into the original convex hull. (Hopefully there's a field with this information already, otherwise you'll need to create it.)