tick marker documentation

Author: mbostock

docs/features/markers.md

@@ -23,6 +23,9 @@ A **marker** defines a graphic drawn on vertices of a [line](../marks/line.md) o
       <option>dot</option>
       <option>circle</option>
       <option>circle-stroke</option>
+      <option>tick</option>
+      <option>tick-x</option>
+      <option>tick-y</option>
     </select>
   </label>
 </p>
@@ -53,6 +56,9 @@ The following named markers are supported:
 * *dot* - a filled *circle* without a stroke and 2.5px radius
 * *circle*, equivalent to *circle-fill* - a filled circle with a white stroke and 3px radius
 * *circle-stroke* - a hollow circle with a colored stroke and a white fill and 3px radius
+* *tick* - a small opposing line
+* *tick-x* - a small horizontal line
+* *tick-y* - a small vertical line
 
 If **marker** is true, it defaults to *circle*. If **marker** is a function, it will be called with a given *color* and must return an [SVG marker element](https://developer.mozilla.org/en-US/docs/Web/SVG/Element/marker).
 

docs/marks/rule.md

@@ -120,16 +120,31 @@ Plot.plot({
 ```
 :::
 
-Rules can also be a stylistic choice, as in the lollipop 🍭 chart below, serving the role of a skinny [bar](./bar.md) topped with a [dot](./dot.md).
+Rules can indicate uncertainty or error by setting the [**marker** option](../features/markers.md) to *tick*; this draws a small perpendicular line at the start and end of the rule. For example, to simulate ±10% error:
+
+:::plot
+```js
+Plot.plot({
+  x: {label: null},
+  y: {percent: true},
+  marks: [
+    Plot.barY(alphabet, {x: "letter", y: "frequency", fill: "blue"}),
+    Plot.ruleX(alphabet, {x: "letter", y1: (d) => d.frequency * 0.9, y2: (d) => d.frequency * 1.1, marker: "tick"}),
+    Plot.ruleY([0])
+  ]
+})
+```
+:::
+
+Rules can also be a stylistic choice, as in the lollipop 🍭 chart below, serving the role of a skinny [bar](./bar.md) topped with a [*dot* marker](../features/markers.md).
 
 :::plot https://observablehq.com/@observablehq/plot-lollipop
 ```js
 Plot.plot({
   x: {label: null, tickPadding: 6, tickSize: 0},
   y: {percent: true},
   marks: [
-    Plot.ruleX(alphabet, {x: "letter", y: "frequency", strokeWidth: 2}),
-    Plot.dot(alphabet, {x: "letter", y: "frequency", fill: "currentColor", r: 4})
+    Plot.ruleX(alphabet, {x: "letter", y: "frequency", strokeWidth: 2, markerEnd: "dot"})
   ]
 })
 ```