Adding the circles to your legend will be some manual work, but adding them to your map is rather easy.
properties of your polygon layer and go to the
symbology-tab. Make sure you have single symbols switched on, not classified or else. You will have some symbology for your area already. From a cartographic point of view it is recommended to only use rather thick outline (e.g. 0.7 to 1 mm) but not to fill the area itself - this avoids informational overflow.
Then use the green plus to add another style and switch this one to
centered- this adds a point to the center of your polygons. Go to the menu which allows you to adapt the marker as much as possible, then click on the rectangular button (with the two triangles) to the far right next to the
size row. There you may choose a column on which to base the size of your point, e.g. your population data. In the next step you need to do some fiddling with how you symbolise it exactly, e.g. multiply or divide your data by some factor, and choose the right units for you points (e.g. you could use pixels as units and base the point-size on
As for the colour of the points, this is rather tricky. I couldn't choose the colour based on a conditional clause, therefore you go back to the marker size and expand the formula to
if("population-column">0,"population-column"/1000,0). This leaves all markers for population changes below zero invisible. Choose e.g. green as a colour for these markers. Then add another marker, use
if("population-column"<0,"population-column"/1000,0) as formula and choose e.g. red.
If you feel really stylish today, you could choose a triangle instead of a circle for your marker - and let it point up or down, based on the development of the population in each area. Formula for this is similar to the ones above:
if("population-column"<0,180,0). Enter this via
edit next to the
Here's an image for your orientation (sorry for the wrong language).
Another word of cartographic warning: If your data is quite diverse, ranging from say 1,000 to 1,000,000, you should rather choose
classified symbology, otherwise you end up with almost invisible points next to ones filling the whole screen.