Symbols must be drawn in size order when they overlap

What it is

When proportional symbols overlap, their draw order determines which observations remain visible. The usual rule is to draw from largest to smallest: large marks go down first and small marks are painted on top. A small location then retains an outline, centre, and interaction target; reversing the order can erase it entirely.

Why it happens

Opaque marks use ordinary painter's compositing: later marks cover earlier ones. Dense locations are often precisely where comparison matters, so a large circle covering a small one can be read as no observation or a zero value. Descending size order does not remove overlap, but it preserves the low-magnitude marks most likely to disappear. Transparency can reveal overlap, yet accumulated opacity creates its own ambiguous dark regions.

Where it stops holding

Ordering preserves part of a symbol, not every circle's full area; under severe overlap, a visible arc is still insufficient for value comparison. Hover, zoom, and an expanded local list help in an interactive map, but they cannot be the only route to reading an exported image or first glance. When layers carry different meanings, an explicit layer priority may override size order.

Applying it

• Sort by value or mapped area in descending order before rendering; use a stable tie-breaker so marks do not jump on refresh.
• Outline small symbols and offer zoom, click-to-expand, or a local list in dense regions.
• Verify a static view of the densest region: every location should retain a recognizable mark, then reconcile that view with the source table for fully hidden entities.

Related

• Same group: U12.04.1 Proportional symbols encode values as symbol area placed at geographic locations · U12.04.2 Symbols must map value to area and carry a size legend · U12.04.4 Proportional symbols fit absolute quantities; choropleths fit rates · U12.04.5 Extreme values shrink the remaining symbols below legibility
• Nearby: U2.06.3 Large bubbles occlude small ones and require size ordering with lower opacity · U12.03.3 Cluster-marker size must represent the number of contained points
• Search terms: symbol occlusion · draw order · proportional symbol map