null filter on the href channel (observablehq#1699)

* null filter on the href channel

closes observablehq#1696

* the title channel is not filtering the mark, unless it's a tip mark (observablehq#1704)

* the title channel is not filtering the mark, unless it's a tip mark

(any mark that depends on this channel must declare it)

* rename to sparse title

---------

Co-authored-by: Mike Bostock <mbostock@gmail.com>

---------

Co-authored-by: Mike Bostock <mbostock@gmail.com>

src/marks/tip.js

@@ -45,6 +45,7 @@ export class Tip extends Mark {
       textAnchor = "start",
       textOverflow,
       textPadding = 8,
+      title,
       pointerSize = 12,
       pathFilter = "drop-shadow(0 3px 4px rgba(0,0,0,0.2))"
     } = options;
@@ -56,7 +57,8 @@ export class Tip extends Mark {
         x1: {value: x1, scale: "x", optional: x2 == null},
         y1: {value: y1, scale: "y", optional: y2 == null},
         x2: {value: x2, scale: "x", optional: x1 == null},
-        y2: {value: y2, scale: "y", optional: y1 == null}
+        y2: {value: y2, scale: "y", optional: y1 == null},
+        title: {value: title, optional: true} // filter: defined
       },
       options,
       defaults

src/style.js

@@ -143,9 +143,9 @@ export function styles(
   mark.shapeRendering = impliedString(shapeRendering, "auto");
 
   return {
-    title: {value: title, optional: true},
-    href: {value: href, optional: true},
-    ariaLabel: {value: variaLabel, optional: true},
+    title: {value: title, optional: true, filter: null},
+    href: {value: href, optional: true, filter: null},
+    ariaLabel: {value: variaLabel, optional: true, filter: null},
     fill: {value: vfill, scale: "auto", optional: true},
     fillOpacity: {value: vfillOpacity, scale: "auto", optional: true},
     stroke: {value: vstroke, scale: "auto", optional: true},