While making a simple SVG chart, I realized I would like to have non-scaling circles for data points. Unfortunately, SVG doesn't support it out of the box.
If you are eager to see the result, jump to the examples below or play with the code on Codepen.
Non-scaling stroke example
SVG does support non-scaling stroke width by setting vector-effect to non-scaling-stroke. When you set it, strokes will stay the same width no matter the scale of the SVG image.
Try resizing your window (On desktop, you should be able to resize each image on it's own by dragging the bottom right corner) , and you'll see that the bottom line always stays 1px, while the upper one scales with the image:
Stroke line cap
SVG also allows you to change how your line's ends look like, by using stroke-linecap property. Available options are butt, square and round, and on the image below they are presented in that order.
<svg aria-label="Example of different stroke-linecap values" style="background: white; padding: 30rem;" viewBox="0 0 200 100" class="image"
As you can see square and round line caps are sticking out of our line. You probably figured it out, but that is exactly what we are going to use to create non-scaling circles and rectangles.
Putting it all together
Up to this point we know that we can have a non-scaling stroke and that we can change its caps to be round and square. If we use a line with no length, sticking parts will join and create a perfect circle (or rectangle). SVG doesn't support lines with length of zero pixels, so we'll set the length to something negligible.
The only thing left is to set a thick stroke width to create a shape.
Circle
The code below will create a black circle with a radius of 50 and center at { x: 50, y: 50 }.
<svg aria-label="Black circle with a radius of 50" viewBox="0 0 100 100" width="200" style="background: white"
<path d="M 50 50 l 0.0001 0" vector-effect="non-scaling-stroke" stroke-width="100" stroke-linecap="round" stroke="black"
Let me quickly break the code down:
d="M 50 50 l 0.0001 0"- this creates a super short line. The first two coordinates (M 50 50) are the line's start and our circle's center. The rest of the code (l 0.0001 0) draws a horizontal line 0.0001px long (Please be aware that going with a lower value might break it in some browsers) .vector-effect="non-scaling-stroke"- enables non-scaling stroke.stroke-width="100"- stroke width should be a diameter (2 * radius) of the circle.stroke-linecap="round"- finally set line caps toroundand by sticking out they will create a circle.
Rectangle
Code for the rectangle is almost the same, just set line caps to square.
Examples
On the left are standard scaling shapes (<circle /> and <rect />) , while on the right are non-scaling shapes created using the method I described above. Try resizing your window and you'll see that the shapes on the right always stay the same size, no matter the image size.
Outlined shapes
We can even add an outline, but we'll need two elements. One acts as the outline, and the smaller one as a fill. Because the stroke width is the diameter, just make sure the smaller element's stroke is smaller for two times the outline width.
To give you an example, if we want a circle with radius of 50px and outline of 1px, stroke widths need to be 100px and 98px.
Which gives us the following code:
This method is practical for elements that should stay the same when image scales. I already mentioned data points on charts, but it is also useful for map markers.
It might even be possible to create more complex elements combining multiple paths, but I haven't really looked into it.
I hope you'll find it useful!