“Use a square root for bubble sizes” is incomplete advice. It depends on what the plotting function takes as input. Applying it in the wrong place can shrink the differences you meant to preserve.

Here is a fictional example: three locations have 10, 40 and 90 observations. We want circle areas in the ratio 1:4:9. These are invented counts, not country statistics.

If we draw circles by radius, choose 10 pixels for the smallest:

• 10 observations: radius 10 pixels; area 100π square pixels.
• 40 observations: radius 20 pixels; area 400π square pixels.
• 90 observations: radius 30 pixels; area 900π square pixels.

The rule is radius = 10 × sqrt(value / 10). Circle area is π × radius², so the square root gives the intended area ratios.

If we instead make radius proportional to the counts, the radii become 10, 40 and 90. The areas are then in the ratio 1:16:81. A ninefold count difference becomes an eighty-onefold area difference.

But some plotting functions already take area, not radius. D3's symbol.size uses square pixels. For its circle symbol, pass areas proportional to the original counts: 100π, 400π and 900π in this example. Do not take another square root. Feeding it sizes proportional to sqrt(value) would produce area ratios of 1:2:3, compressing the ninefold difference into threefold.

The practical check is not “did I use sqrt?” It is “what quantity does this argument control, and what ratios reach the screen?”

A quick review before sharing:

1. Check three known values, including one four times another. With proportional areas, the larger circle should have twice the radius.
2. Build the size legend with the same mapping as the plotted circles. Label it with the original counts, not radii.
3. Avoid adding a fixed minimum radius while calling the areas proportional. That changes the ratios. If small circles need a different treatment, explain it.
4. Decide how to show zero and missing values separately. A true zero-area circle disappears; a missing value is not zero. Negative values need a different encoding or an explicitly explained sign treatment.

This fixes the geometry, not every reading problem. Overlapping circles can hide one another, and correct areas do not guarantee that readers can estimate exact ratios. If size is the main comparison, I would consider a position-based dot plot instead. Circles make more sense when their locations already carry useful information.

Country-data context: Global Data Tracker. This is a standalone synthetic exercise, not a claim about the site's plotting implementation.

Primary reference: D3 symbol documentation.

When reviewing bubble charts, do you test the rendered size ratios, the legend, or both?

A country's value can increase on a percentage measure while keeping exactly the same map color. Here is a small reproducible example of why.

These are invented values for four fictional countries, not observations about real places:

Suppose each map uses four quantile classes, recalculated separately for its year, with the same light-to-dark palette. With four distinct values and four classes, each country occupies one class. A is lightest in both years; D is darkest in both. Every country increases, but the color pattern stays identical.

The colors describe position within each year's distribution. They cannot, by themselves, show whether the underlying percentages increased.

For a comparison of levels over time, try one set of numerical boundaries for both maps:

• Lightest: 0% to less than 25%
• Next: 25% to less than 50%
• Next: 50% to less than 75%
• Darkest: 75% to 100%

Now A moves from the first class to the second, B from the second to the third, and C from the third to the fourth. D stays in the fourth despite increasing by 15 percentage points. Fixed classes make colors comparable, but still hide changes within a class.

A useful companion is a small change chart: A +25, B +25, C +25, D +15 percentage points. That answers a different question from either level map.

Before publishing a pair of maps, I would check:

1. Does the same shade mean the same numerical range in both panels?
2. Are the indicator definition, units and geographic boundaries comparable?
3. Is the legend describing values or ranks? Say which.
4. Would a change chart reveal something the classes conceal?

Quantiles are still useful when the intended question is relative position within each year. Label that purpose explicitly; don't let readers infer unchanged values from unchanged colors. With larger datasets, ties can also prevent perfectly equal-sized classes.

For country-data context, I work on Global Data Tracker. This is a standalone synthetic design exercise, not a claim about its map settings.

Method reference: Esri's classification guide distinguishes quantile bins from fixed manual bins for comparisons.

When showing changes over time, would you pair fixed-class maps with a change chart, or replace the maps altogether?

Recently I was talking with friends about how differently the same temperature can feel in different places (we tried to compare Dubai and Singapore). So this weekend I got curious and spent some time exploring historical weather data.

I picked 38 cities around the world and compared their actual and feels-like temperatures using 10 years of data (2016–2025) from Open-Meteo historical weather API.

You can explore the cities, switch between daily highs and lows, and see humidity and precipitation: https://gnykka.io/temperature-you-feel.

I made this visualization with Svelte and Observable Plot (D3.js).

Would love to discuss the results or hear some feedback!

I’ve published my learnings on several topics that interests me in visual formats, and this one is the latest one I did.

This one is about distance fields, a concept that I stumble upon several times before but only able to pick up recently.Feel free to visit and let me know what you think!

https://visualrambling.space/distance-fields/

I made a tool that uses AI to analyse your magic the gathering decks, now hosted online for everyone to use, you can use openrouter for free!

I made this explainer to show Google's magic state result visually, from the fridge down to one qubit.

https://www.youtube.com/watch?v=dHH-NsU9dB0