Creating graduated classes for election swing map in QGIS

Question

I am using QGIS right now to create an election swing map. This is different from a usual election map in that it measures differences in winning margins from one election to another. For example, Joe Biden could have won a precinct by 100 points in 2020, after Trump beat Clinton in that same precinct by 100 points. That would be a swing of D+200.

Well, even though D/R+200 is the maximum possible swing, some precincts never reach that level. This complicates my graduated classes.

I am using red to show a Republican shift, blue to show a Democratic shift, and white to show no shift at all. However, because the maximum shift to the Republicans is R+200 and the maximum shift to the Democrats is only D+150, this makes an unbalanced scale. A +0 shift is now on the red side of the color scale, when it absolutely needs to be clear white.

I have used all the other modes (equal interval, logarithmic, etc.), and none are able to create the color scale I want. I'd like to make 999 classes, separated equally, only that after D/R+100, it would go on to infinity.

Above, this scale is what I am aiming for, but I'd like it to be "-INFINITY - 99.799800," with the same on the Democratic/blue side of the scale. This way, the scale would be even with +0 being white, and any swing above D/R+100 would be capped off.

Answer 1

You have a range from -200 to 150 and you want the white color to be at 0. In your color ramp, white is at 50%, thus exactly in the middle - thus for your range of values at -25 (mean of -200 and 150).

So what you can do is shifting the white tab of your color ramp to the right. In the range from -200 to 150, 0 is at 57.14% (200/150*100) of the total range. So click on the color ramp, select the tab of the white color and manually change its relative position to 57.1%.

Consider deleting all the other color position tabs (except the first/left/red one and the last/right/blue one) to get a continuous color ramp.

All but the first, middle and last tab deleted:

Answer 2

Another simple solution would be to use the Assistant in your Fill color.

This sample layer has a field with values from -200 to 150. As you can see, a normal graduated symbology would output your same results:

Instead, you can just symbolize using Single Symbol and use the Assistant to modify your Fill Color:

From here you can populate the min - max values as you need them to be, the scale is independent from your values. In your case, you want a range from -200 to 200:

You can do this even if there are no values from 150 to 200. Make sure to not apply any transformation curves since you want a linear scale.