document crosshair

Author: Fil

docs/interactions/crosshair.md

@@ -1,21 +1,106 @@
+<script setup>
+
+import * as Plot from "@observablehq/plot";
+import * as d3 from "d3";
+import alphabet from "../data/alphabet.ts";
+import aapl from "../data/aapl.ts";
+import penguins from "../data/penguins.ts";
+
+</script>
+
 # Crosshair mark
 
-The **crosshair mark** …
+The **crosshair mark** shows the *x* (horizontal↔︎ position) and *y* (vertical↕︎ position) value of the point closest to the [pointer](./pointer.md) on the bottom and left sides of the frame, respectively.
+
+:::plot defer
+```js
+Plot.plot({
+  marks: [
+    Plot.dot(penguins, {x: "culmen_length_mm", y: "culmen_depth_mm", stroke: "sex"}),
+    Plot.crosshair(penguins, {x: "culmen_length_mm", y: "culmen_depth_mm"})
+  ]
+})
+```
+:::
+
+If either **x** or **y** is not specified, the crosshair is be one-dimensional.
+
+:::plot defer
+```js
+Plot.plot({
+  marks: [
+    Plot.tickX(penguins, {x: "body_mass_g"}),
+    Plot.crosshair(penguins, {x: "body_mass_g"})
+  ]
+})
+```
+:::
+
+For charts which have an independent variable, typically a time series with dates on the *x* axis, the crosshairX mark is recommended to track the horizontal position of the pointer.
+
+:::plot defer
+```js
+Plot.plot({
+  marks: [
+    Plot.lineY(aapl, {x: "Date", y: "Close"}),
+    Plot.crosshairX(aapl, {x: "Date", y: "Close"})
+  ]
+})
+```
+:::
+
+Conversely, for charts where the independent variable is along the *y* axis, the crosshairY mark is recommended to track the vertical position of the pointer.
+
+:::plot defer
+```js
+Plot.plot({
+  marks: [
+    Plot.barX(alphabet, {x: "frequency", y: "letter", fillOpacity: 0.25, sort: {y: "-x"}}),
+    Plot.crosshairY(alphabet, {x: "frequency", y: "letter", ruleStrokeOpacity: 1})
+  ]
+})
+```
+:::
 
 ## Crosshair options
 
 The following options are supported:
 
-…
+* **maxRadius** - reach of the crosshair pointer; defaults to 40 pixels
+* **ruleStroke** - the rule color
+* **textFill** - the text fill color
+* **textStroke** - the text stroke color; defaults to *white* to improve legibility
+* **color** - shorthand for setting both **ruleStroke** and **textFill**
+* **ruleStrokeOpacity** - the rule stroke opacity; defaults to 0.2
+* **ruleStrokeWidth** - the rule stroke width; defaults to 1
+* **ruleStrokeOpacity** - the rule stroke opacity; defaults to 0.2
+* **textStrokeOpacity** - the text stroke opacity; defaults to 1
+* **textStrokeOpacity** - the text stroke width; defaults to 5
+* **dx** - the horizontal shift of the text relative to the *y* axis, in pixels; defaults to -9
+* **dy** - the vertical shift of the text relative to the *x* axis, in pixels; defaults to 9
+
+
 
 ## crosshair(*data*, *options*)
 
 ```js
-Plot.crosshair(flare, {path: "name", delimiter: "."})
+Plot.crosshair(cars, {x: "economy (mpg)", y: "cylinders"})
 ```
 
-Returns a new crosshair mark with the given *data* and *options*.
+Returns a new crosshair for the given *data* and *options*, drawing horizontal and vertical rules centered at the point closest to the [pointer](./pointer.md#pointeroptions). The corresponding **x** and **y** values are also drawn just outside the bottom and left sides of the frame, respectively, typically on top of the axes. If either **x** or **y** is not specified, the crosshair will be one-dimensional.
 
 ## crosshairX(*data*, *options*)
 
+```js
+Plot.crosshairX(aapl, {x: "Date", y: "Close"})
+```
+
+Like crosshair, but uses the [pointerX](./pointer.md#pointerxoptions) transform: the determination of the closest point is heavily weighted by the *x* (horizontal↔︎) position; this should be used for plots where *x* represents the independent variable, such as time in a time-series chart, or the aggregated dimension when grouping or binning.
+
 ## crosshairY(*data*, *options*)
+
+```js
+Plot.crosshairY(alphabet, {x: "frequency", y: "letter"})
+```
+
+Like crosshair, but uses the [pointerY](./pointer.md#pointeryoptions) transform: the determination of the closest point is heavily weighted by the *y* (vertical↕︎) position; this should be used for plots where *y* represents the independent variable, such as time in a time-series chart, or the aggregated dimension when grouping or binning.