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I need to create a proportional symbol map for a GIS class. I am wondering, how can I manually change the symbology to make the dots grow in size depending on their magnitude represented, and also how can I manually change the color scheme?

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There is a very simple solution to this. In QGIS, double click on the layer you are dealing with. Then go to the Style menu. In the Style menu, set your classification to “Graduated”.

Figure1

Then, double click on your symbol. This will take you to a menu where you have the option of changing your symbol layer type from “Simple fill” to “Centroid fill”.

Figure2

Once you have done that, see the symbol layers bar in on the left and click Marker. Click “circle” for your symbol type. Then change the size and color of your circle accordingly. Simply repeat for your other symbols.

Figure3

Let me know if that helped and if you were successful! (Other) Olivia

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For those using newer versions of qgis (2.12.2 Lyon) in Style -> Graduated change the Method drop down from "Color" to Size. You can change the symbol properties like colour and shape and the size based classes will adjust accordingly

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    your solution applies only to point layers, not polygons...
    – Gerardo Jimenez
    Jun 20, 2016 at 15:56
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    So to have graduated symbols (not proportional), an extra step is required : create a point layer by extracting polygons centroïds.
    – sigeal
    Oct 9, 2018 at 10:05

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