code4fukui.github.io/csv-map/csv-map.jsのL.marker()をL.polygon()にするサンプルです。

csv-map.js.diff

--- csv-map.js.ORIG	2021-08-14 17:39:28.172959000 +0900
+++ csv-map.js	2021-08-14 17:40:28.675902000 +0900
@@ -156,8 +156,6 @@
         }
       }
 
-      const marker = L.marker(ll, opt);
-
       const d2 = (() => {
         if (!filter) {
           return d;
@@ -171,7 +169,36 @@
         return res;
       })();
       const tbl = makeTable(d2);
+
+      let a1 = d2['河川水位'];
+      let a2 = d2['はん濫危険水位'];
+
+      if (String(a1).match(/^[\.0-9]*$/) == null) {
+        a1 = 0.01;
+      }
+      if (String(a2).match(/^[\.0-9]*$/) == null) {
+        a2 = a1;
+      }
+
+      const rate = 0.01 * (a1 / a2);
+      const marker = L.polygon([
+        [ll[0] + 0.01, ll[1] - 0.01],
+        [ll[0] + 0.01, ll[1] + 0.01],
+        [ll[0],        ll[1] + 0.01],
+        [ll[0],        ll[1] - 0.01],
+      ], {});
+      const marker2 = L.polygon([
+        [ll[0] + rate, ll[1] - 0.01],
+        [ll[0] + rate, ll[1] + 0.01],
+        [ll[0],        ll[1] + 0.01],
+        [ll[0],        ll[1] - 0.01],
+      ], {
+        color: 'red',
+        fillColor: 'red'
+      });
+
       marker.bindPopup((title ? (url ? `<a href=${url}>${title}</a>` : title) : "") + tbl);
+      this.iconlayer.addLayer(marker2);
       this.iconlayer.addLayer(marker);
       lls.push(ll);
     }