Legend omits rivers mapped as areas

[1ec5]

The legend has entries for “River” and “Intermittent river” based on waterway=river ways, depicting them as lines. This can be confusing when all that’s visible in the current viewport is a river that’s been mapped as a natural=water water=river area, zoomed in far enough to be styled as an area. The legend is designed to be intuitive to non-mappers, to whom differences in mapping style would be irrelevant, realistically speaking.

We could add a separate entry for river areas (and another one for intermittent river areas), but I’m unsure how to describe it in lay terms. “River area” sounds like the flood protection area around a river. “Major river” would be misleading because every river can be mapped as an area and shown as such at a high enough zoom level. We could take a page out of publishing industry practices and pretend we’re exercising editorial control by labeling the entry “Selected rivers” – except that all the legend entries are singular for simplicity.

A more robust approach would be to enable a single entry to feature both a swatch and a line side by side. We’d probably need to convert the swatch template from an HTML table cell to an embedded SVG, just like the line template. That would give us the ability to make the swatch a different shape, such as a blob (for lakes) or a wavy box bordered on only two sides (for river areas).

Similar considerations will apply when we implement roadway surfaces: #486.

/ref #671 (comment)